In her first professional match in almost half a year, world number two Halep, who skipped last week's Palermo Open and will not make a decision on whether to play the US Open or not until after the tournament in the Czech Republic, was made to work hard for victory by the unseeded Hercog.
The Romanian picked up an early break and responded to dropping her serve in game three of the first by reeling off the next four in succession to take the set.
When Halep started the second set by breaking Hercog the writing appeared to be on the wall, but the Slovenian world number 46 fought back valiantly to force a decider in emphatic fashion.
Halep was unable to convert any of the six match points that came her way after breaking for a 5-3 lead, with Hercog clinging on to take the match to a third-set tie-break.
After failing to consolidate an opening mini-break, Hercog's resistance was finally broken after two hours and five minutes when a forehand into the net sent top seed Halep into the second round.
Wildcard Barbora Krejcikova ensured Halep's next match would not be an all-Romanian affair by beating Patricia Maria Tig 6-4 6-3.
Second seed Petra Martic defeated Varvara Gracheva 7-6 (7-2) 6-3 and third seed Elise Mertens needed three sets to overcome Jasmine Paolini 7-5 4-6 6-3.
A number of other seeded players were not as successful. Ekaterina Alexandrova (5) fell to qualifier Lesia Tsurenko, while Anastasia Pavlyuchenkova (6) went down 7-5 6-2 to Arantxa Rus, and Anastasija Sevastova (9) lost 6-2 6-2 to Irina-Camelia Begu.
World number two Halep claimed a 7-6 (7-2) 6-3 victory in the Czech capital and has the chance to extend her winning streak to nine matches having won the Dubai Duty Free Tennis Championships in February before the WTA Tour's suspension for the coronavirus pandemic.
Begu deserves immense credit, though, having spent seven hours on court over her first three matches and having to conclude a three-set quarter-final triumph over Sara Sorribes Tormo earlier in the day with rain having delayed the match.
"I didn't play my best tennis but also she was tired, so in my opinion, it was a good match," Halep said in her on-court interview.
"I'm really happy that I could win it. It's always tough to play against her. She's a great player."
Awaiting in Sunday's final is third seed Elise Mertens, who defeated former finalist Kristyna Pliskova 7-5 7-6 (7-4) in the day's earlier last-four contest.
Halep, appearing for the first time since a run to the last four at the Australian Open, lost to Sabalenka in their most recent meeting - in Adelaide in January - but had won her prior two encounters with the world number 13.
Despite trailing in the first set, the world number two rallied in the second thanks to three breaks of serve, before cruising in the decider to wrap up a 3-6 6-2 6-2 win over her Belarusian opponent.
The Romanian, a Dubai champion in 2015, will now go up against Jennifer Brady, whose run of upsets continued as she overcame Australian Open finalist Garbine Muguruza to claim her third straight top-20 triumph.
Having already knocked out third seed Elina Svitolina and Marketa Vondrousova in previous rounds, Brady moved into the semis with a 6-7 (5-7) 6-3 6-4 win.
It is the first time in Brady's career that the American has made it into the last four of a WTA Premier event, though she was not fully satisfied with her performance.
"I wasn't feeling my best," she said in quotes reported by the WTA's official website. "Maybe I didn't play my best tennis, but I just found a way to stay in there, compete and come out on top."
The other semi-final will see Elena Rybakina, whose fine run continued with a win over Karolina Pliskova, meet Petra Martic.
Dubai debutant Rybakina has started 2020 in fine form and dispatched world number three Pliskova on Thursday, completing a 7-6 (7-1) 6-3 success in one hour and 41 minutes.
It means Rybakina has made her fourth semi-final in five tournaments so far this year, with a tie against Martic - who beat Anett Kontaveit 7-6 (7-4) 6-1 - her reward.
The 2018 champion at Roland Garros suffered a tear in her left calf while playing against Angelique Kerber at the Internazionali BNL d'Italia in Rome last week.
Halep posted a statement on Twitter on Friday confirming she will not recover in time to take part in the second grand slam of the season, which begins in Paris on May 24.
"It's with a heavy heart that I announce my withdrawal from Roland Garros this year,” the world number three said.
"Unfortunately the tear in my left calf muscle needs more time to recover and the timeline is just too short.
"Withdrawing from a grand slam goes against all my instincts and aspirations as an athlete, but it is the right and only decision to make.
"The thought of not being in Paris fills me with sadness, but I will focus my energy on recovery, staying positive and getting back on court as soon as it is safe to do so.
"Roland Garros 2022, I am coming for you! A bientot [see you soon!]"
Halep defeated Sloane Stephens to win the clay-court tournament three years ago, having previously lost finals in 2014 and 2017.
The 29-year-old Romanian – who also won Wimbledon in 2019 – made the quarter-finals at the Australian Open back in February, her run in Melbourne ended by Serena Williams.
Third seed Halep was the highest-ranked player in action on Wednesday but came unstuck against Mertens despite taking the opener and leading by a break in each of the subsequent sets.
Mertens emerged a 4-6 7-5 7-5 winner after two hours and 34 minutes, defeating Halep for the first time on clay and only the second time on any surface.
There were opportunities for both players throughout and Halep struck first, losing her break lead in the first but recovering on this occasion.
The Romanian, a two-time winner who has also twice lost in the final in Madrid, twice cancelled out a deficit in the second but could not then capitalise on a lead of her own.
Mertens battled back and broke to love to level the match, then maintained her composure to see out a back-and-forth decider once Halep had again forged ahead.
"She played really well, and she definitely deserved to win because she was stronger in the end," Halep said.
"I cannot say I played bad, but I did some mistakes, important mistakes. Sometimes it's just a little bit and the match is going away."
Sabalenka is up next for Mertens, although she had a much more straightforward path through the third round.
Jessica Pegula held her up for only 52 minutes in a dominant 6-1 6-2 success.
In the same half of the draw, Anastasia Pavlyuchenkova upset another seed, following up her straight-sets victory over Karolina Pliskova by beating Jennifer Brady in three. She plays Karolina Muchova next.
At L'Open 35 de Saint-Melo, second seed Rebecca Peterson came from behind to see off Xiyu Wang, while Tamara Zidansek and Arantxa Rus also produced comebacks.
Nina Stojanovic survived three sets, but Christina McHale crashed out to Viktoriya Tomova.
Tournament organisers have asked government minister Roberto Speranza to make an exception after quarantine restrictions were announced for all visitors who have spent time in Romania or Bulgaria in the past 14 days.
Halep, who has spent lockdown at home in Romania, has yet to make a public comment on the possibility of being ruled out of the clay-court event, which starts on August 3.
The Palermo Open will be the first WTA Tour tournament since March and Halep is the star attraction in the field.
Italy, one of the first nations to be badly hit by the coronavirus pandemic, is determined to be cautious about visitors arriving from other countries.
Tournament chief executive Oliviero Palma is waiting to hear back from the authorities after sending what he described as an "urgent letter" to Speranza.
Palma wrote on Twitter: "We wrote a letter to the Minister because we're convinced that the health protocols adopted by the WTA are so strict to guarantee the safety and health not only of athletes, yet also of all the various workers involved in the event.
"The provision would penalise a player like Simona Halep, [the] world's number 2 and Wimbledon's reigning champion, who wouldn't take part in Palermo's tournament anymore."
Speranza said on Saturday: "We must protect our country within an international context that is worsening. We live on one planet. The battle is won with determination and solidarity."
Top seed Halep came through a testing quarter-final with Aryna Sabalenka, but her last-four match was significantly more straightforward on Friday.
Halep blitzed through the encounter in just 62 minutes, the 2015 champion appearing way more at ease than in the pair's first-round clash at the Australian Open in January, when Brady succumbed 7-6 (7-5) 6-1.
World number two and two-time grand slam champion Halep faced only two break points in the entire match and she saved both, while she also hit 16 winners to eight unforced errors in a performance she felt was almost perfect.
"It's fun when you feel the ball and when you can do anything you want," Halep said. "Today it was one of those days, I could open the court easily. I felt like every time I hit the ball, I feel it.
"It's always a nice opportunity and privilege to play the last match of the tournament. I play for another trophy, so I'm relaxed. I'm focused. I really want it."
Elena Rybakina will be Halep's next opponent, with the Kazakh securing progress to a fourth final already this year.
Rybakina showed her resilience to come through a gripping duel with eighth seed Petra Martic, saving set points in both sets before eventually winning 7-6 (7-5) 7-6 (7-2).
The 20-year-old has already beaten Australian Open champion Sofia Kenin and former world number one Karolina Pliskova in Dubai, and now she is aiming for the biggest scalp of the lot – and Halep knows she will need to be aggressive to upset the upstart's rise.
"It's amazing what [Rybakina's] done already this year," Halep added. "I saw her a little bit during some matches.
"She has a big serve. I have to pay attention on my return and to be like today: confident, aggressive if there is a chance, just fighting. It's the best thing to do now."
Halep opted against travelling to New York for the US Open last year due to the pandemic before testing positive for COVID-19 last October.
The two-time grand slam champion missed the French Open and Wimbledon this year with a torn calf muscle, but claimed her first win since May at the Western and Southern on Tuesday, beating Magda Linette 6-4 3-6 6-0.
Former world number one Halep revealed she feels much more comfortable back on the circuit after taking the vaccine.
"I don't feel afraid, because I am vaccinated," The Romanian told reporters. "I feel protected. I am taking care. I wear the mask when many people are around."
Players have faced strict protocols since the tour resumed, with the challenges of staying in biosecure bubbles and playing in empty stadiums last year before restrictions gradually eased.
Stefanos Tsitsipas this week said that he would only take the vaccine if it becomes mandatory to play on the ATP Tour after Novak Djokovic stated he hopes it will not become a requirement to take the jab.
Halep hopes fellow professionals take the vaccine so they do not face such strict protocols.
"I don't like the bubbles," Halep said. "I feel very stressed when I'm in the bubble.
"So if people can vaccinate more and more (it) will be better, because we will not have more restrictions anymore."
Second seed Halep is hunting her first appearance in a Stuttgart final, and she looked in fine form as she took just 56 minutes to dismiss Vondrousova 6-1 6-3.
Vondrousova – the 2019 French Open runner up – had won both of her previous meetings with Halep, but on this occasion she was no match for the world number three, who has won every major European clay-court tournament other than Stuttgart.
"Starting the clay-court season makes me very happy and motivated, extra motivated actually," Halep said in a post-match news conference.
"I missed playing matches. I had a few weeks without an official match and it was kind of tough when I started the match, but my mind was strong enough just to give focus on what I have to play. So, I did it great and today it was a great match.
"I was a little bit nervous before the match because I played twice against her and she beat me every time.
"It is important for my mind, for my game, because the clay-court season is going to be very tough. Now this match gave me confidence that my level is high, and if I keep doing the same thing, I will be good."
Facing Halep in the quarter-finals will be Ekaterina Alexandrova, who defeated world number 12 Belinda Bencic, and joining them in the final eight is Elina Svitolina, who defeated Angelique Kerber 7-6 (7-4) 6-3.
Two-time Stuttgart champion Kerber and Svitolina had faced off 13 times before Thursday's contest, and the Ukrainian is now 9-5 up in that head-to-head tussle.
While Aryna Sabalenka also progressed, Karolina Pliskova teed up a clash with top seed Ash Barty by coming from behind to beat Jelena Ostapenko 6-7 (7-9) 6-4 6-3.
At the Istanbul Open, top seed Elise Mertens also needed three sets to forge a path into the last night.
Mertens had to show her mettle in a contest which lasted over two hours, but she eventually got the better of Viktorija Golubic 6-4 4-6 6-1.
Veronika Kudermetova also made the last eight, as did Ana Bogdan, who fended off Barbora Kreijcova, while Fiona Ferro defeated Vera Zvonareva.
Second seed Badosa had no answer to the brilliance of Halep in her homeland as the relentless Romanian won 6-3 6-1 at the Caja Magica.
Unseeded two-time grand slam champion Halep took just an hour and 17 minutes to eliminate Badosa, breaking three times in the first set and twice in the second.
Halep, in a new era with Patrick Mouratoglou in her corner, won 84 per cent of points when she landed her first serve in, staying in the hunt to win this tournament for a third time with the French Open on the horizon.
The former world number one will face Coco Gauff for a place in the quarter-finals following the American's 6-1 2-6 6-4 late-night defeat of Yulia Putintseva.
Gauff, the 14th seed, racked up 10 double faults and was pushed all the way by Putintseva, but served out the match after securing a third break in a tight final set.
Ons Jabeur earlier advanced into the third round at the expense of Varvara Gracheva, recovering from a second-set bagel to win 7-5 0-6 6-4.
Jabeur faces a meeting with Belinda Bencic after the Swiss dispatched Karolina Muchova 6-3 4-6 7-5.
Victoria Azarenka came from a set down to beat Tamara Zidansek 3-6 6-1 6-3 to set up a meeting with Amanda Anisimova, who also had to come from behind to dispatch Petra Martic 3-6 6-3 6-2.
Ekaterina Alexandrova and Marie Bouzkova also progressed in the Spanish capital.
World number two Halep had to come from a set down against Rybakina, who put in a wonderful display and came within a break of securing her second title of 2020.
But the 20-year-old ultimately fell short in a final-set tie-breaker as Halep triumphed 3-6 6-3 7-6 (7-5) after two hours and 29 minutes on Saturday.
Halep, a Dubai champion in 2015, saved a break point in just the second game of the match but could not keep Rybakina at bay when the next opportunity came, going long in the sixth to fall behind.
Rybakina saw off two late openings to claim the set, only for Halep to make a brilliant start to the second and soon secure a trio of opportunities, breaking to love when Rybakina failed to pick out the left corner of the court.
Halep was made to work to consolidate the break but then had another two break points in the next game.
However, she passed up both opportunities, allowing Rybakina to roar back and break to love, with Halep signalling for her coach after going wide at the end of the rally.
Halep got the motivation she needed and chances came thick and fast in a mammoth game, which she eventually claimed en route to levelling the match when Rybakina slowed and a fourth break point was taken.
Momentum in the decider similarly swung back and forth, with Rybakina in control when Halep double-faulted before that advantage was immediately cancelled out.
A stunning volley put Rybakina 5-4 up, but Halep rallied to hold serve before breaking in the next game to take a 6-5 lead.
Yet Rybakina was not beaten and, at the second attempt, broke serve to force the tie-break.
Rybakina squandered the chance to take a 6-4 lead in the tie-break on her own serve, though, and Halep duly took advantage, with an overhit return handing the Romanian her second triumph in Dubai.
Neither player was particularly convincing in the early stages, with each of the first four games going against serve, but it was Krejcikova who settled quicker to take the first set 6-3.
She then brought the second set back to 5-5 after being 5-2 down, but Halep managed to see it out to level the match, before going on to seal the 3-6 7-5 6-2 win in the decider.
Top-seed Halep will face Poland's Magdalena Frech – ranked 174 in the world – in the quarter-finals after she saw off Arantxa Rus 6-3 2-6 6-4.
The day began with what turned out to be a gruelling encounter between Eugenie Bouchard and Tamara Zidansek, but the Canadian – who is attempting to climb back up the rankings after slipping to 330 – prevailed to reach her second WTA quarter-final of the year, winning 7-6 (7-2) 6-7 (2-7) 6-2 in two hours, 56 minutes.
Sara Sorribes Tormo had no such difficulty against Laura Siegemund as she emerged 6-2 6-3 victor, but she will have to wait until Friday to find out her next opponent after Irina-Camelia's meeting with Leonie Kung was suspended due to poor light.
Kung had just won the first set 7-6 (7-3).
Tournament favourite Halep eventually looked comfortable in her 7-5 6-4 against Dayana Yastremska, but the Romanian had to fight back from a break down in the opening set as she was put on the back foot.
Halep slid 3-0 down in the opener, but she produced an admirable response when winning 12 of the following 13 points as she levelled up.
Another break from 5-5 allowed Halep to take charge, and from there she rarely looked in trouble, remaining in the ascendancy early in the second as she went 3-0 up, ultimately seeing out the win professionally despite Yastremska giving herself a chance with a break of her own.
"It was not easy against her because you don't really have the rhythm. She hits it very strong and it's tough to return her balls," Halep said, highlighting the need to get her head straight after the early wobble.
"After I lost the first three games, I just had to focus a little bit better and stop giving her the balls that she likes.
"I tried to change it up a little bit, make her move, [hit shots] a little bit high. I think during the match I found some solutions that were good to win.
"Big picture, I think it was a great match and gives me confidence that even in these conditions with a big hitter that I could win in two sets."
Awaiting Halep is a meeting with Yulia Putintseva, who fought back from a set down against fellow Russia-born Kazakh Elena Rybakina to edge an absorbing contest 4-6 7-6 (7-3) 6-2.
Second seed and defending champion Pliskova came through fairly comfortably against Anna Blinkova 6-4 6-3, with the Czech next going up against Elise Mertens, who saw off Danka Kovinic in similarly routine fashion with a 6-4 6-4 victory.
Another tantalising quarter-final will be contested between Victoria Azarenka and Garbine Muguruza. Spaniard Muguruza caught the eye as she beat seventh seed Johanna Konta 6-4 6-1, not dropping serve once against her British opponent.
Azarenka, who incredibly crushed Australian Open champion Sofia Kenin without dropping a game on Thursday, should be fresh for the quarter-final as her last-16 opponent Daria Kasatkina retired due to an ankle injury at 6-6 in the first set of their contest.
Halep was on the brink of a surprise defeat to the in-form world number 45, but hung in there to win a thriller 1-6 6-2 7-6 (9-7) on Wednesday.
The two-time grand slam champion and Jabeur were given great backing from passionate Romanian and Tunisian supporters in a pulsating contest that swung one way and the other.
Halep appeared destined to complete the turnaround with ease when she led 3-0 in the final set, but the tenacious Jabeur was relishing the battle and showed great spirit to take the upper hand at 6-5.
The unseeded Jabeur was broken to love when serving for the match, though, and a double-fault gifted 2015 champion Halep a 4-2 lead in the tie-break, yet there was more drama to come.
Jabeur fended off three match points to earn one of her own, but Halep showed her grit to win the next two points before her gutsy opponent drilled a forehand long to end a tense two-hour contest.
Halep will face Aryna Sabalenka in the last eight after the seventh seed defeated Elise Mertens 6-4 6-3.
Australian Open runner-up Garbine Muguruza advanced with a hard-fought Veronika Kudermetova, winning 7-5 4-6 6-4 two days after knocking out the returning Kim Clijsters.
Elena Rybakina, a three-time finalist this year, got the better of Katerina Siniakova for the second successive week, a 6-3, 6-3 victory setting up a meeting with Karolina Pliskova.
Second seed Pliskova lost just three games in emphatic defeat of Kristina Mladenovic, while Petra Martic, Jennifer Brady and Anett Kontaveit were also victorious.
Spectators were allowed at a WTA Tour event for the first time since the coronavirus pandemic suspended competitions, the Rome crowd witnessing Halep defeat Garbine Muguruza 6-3 4-6 6-4 in two hours and 14 minutes.
Muguruza was the last woman to beat Halep, in the semi-finals of the Australian Open in January, but she was unable to produce a repeat in Italy, where the Romanian will be hoping to add to the recent titles she won in Abu Dhabi and Prague either side of the break.
Halep, a two-time runner-up at this tournament, was 5-1 up in the third set but was pegged back to 5-4, only for Muguruza to produce back-to-back double faults that handed her opponent the victory.
"The victory is really important against Muguruza, always," Halep said afterwards.
"She's a very, very difficult player and a good player. It's always a good vibe and feeling when I win against her.
"I feel confident now that I was able to win this match."
Next up for Halep is Karolina Pliskova after the defending champion overcame Marketa Vondrousova 6-2 6-4.
Pliskova raced into a one-set lead against her fellow Czech, who was the runner-up at Roland Garros last year, but dropped her serve at the start of the second.
Vondrousova was unable to consolidate and paid the price for failing to stay in the rallies with the former world number one in the closing stages of a match that lasted one hour and 22 minutes.
Former world number one Halep had to be helped off court by coach Darren Cahill after she was injured while leading Angelique Kerber 6-1 3-3 in the second round of the Internazionali d'Italia on Wednesday.
The two-time grand slam champion on Friday announced the extent of her injury. Her misfortune comes with just over two weeks remaining until the clay-court grand slam at Roland Garros gets under way on May 30.
Wimbledon is the next major on the calendar after the Paris slam, with the All England Club tournament scheduled to start on June 28 in London.
"After an MRI here in Rome I can confirm that I have small tear high up in the left calf," Romanian Halep posted on her social media accounts.
"I will fly home today and begin recovery in the pool and gym on Monday.
"I'm staying positive and will do everything i can to speed up my return."
Halep, now 29 years old, won her first major title in Paris three years ago and went on to be crowned Wimbledon champion in 2019.
Ruse won her first WTA title at the Hamburg European Open in July and reached the final of the Palermo Ladies Open later that month, but she was second best against Halep.
The top seed produced a commanding performance as she held serve throughout and broke her opponent twice in both sets to progress 6-1 6-2 in a time of 73 minutes.
Halep, who departed at the quarter-final stage of the Kremlin Cup last week, is on course to meet US Open winner Emma Raducanu in the semi-finals in Cluj.
Anett Kontaveit also prevailed on Wednesday with a straight-sets win of her own, the second seed seeing off Aleksandra Krunic 6-3 7-5.
The 25-year-old, seeking her third WTA 500 indoor trophy in the space of two months, made the most of some Krunic errors to set up a last-16 tie with Alison Van Uytvanck.
Anhelina Kalinina earlier beat Anna-Lena Friedsam 6-2 6-2 to become the first player to book their place in the quarter-finals.
At the Courmayeur Open in Italy, top seed Dayana Yastremska required three sets to overcome Kateryna Kozlova 6-1 3-6 7-5.
Clara Tauson and Ann Li also avoided upsets as they beat Giulia Gatto-Monticone and Kamilla Rakhimova respectively.
New restrictions were announced on Friday for all visitors who have spent time in Romania or Bulgaria in the past 14 days; however, tournament organisers are confident they will not apply to athletes.
Tournament chief executive Oliviero Palma wrote to Italy's health minister Roberto Speranza to seek an exemption, but on Saturday he sounded an optimistic note regarding the reigning Wimbledon champion.
"Simona Halep should participate in the 31st Palermo Open," Palma said.
"Following a literal interpretation of the provisions in force, it seems that workers, therefore professional athletes too, should be exempted from the mandatory quarantine.
"We are waiting for an official clarification from the competent authorities, but we are confident. I reassured Simona Halep's manager."
World number two Halep, who has spent lockdown at home in Romania, is due in Sicily for the event which starts on August 3.
The Palermo Open will be the first WTA Tour tournament since March and Halep is the star attraction in the field.
Italy, one of the first nations to be badly hit by the coronavirus pandemic, is determined to be cautious about visitors arriving from other countries.
Speranza said on Saturday: "We must protect our country within an international context that is worsening. We live on one planet. The battle is won with determination and solidarity."
Halep, who was the year-end number one in 2017 and 2018, has been coached by Cahill for the past six years.
She won the 2018 French Open, her first grand slam title, under his guidance before Cahill left the team for a spell to spend more time with his family.
Cahill – who also guided Halep to the 2018 Australian Open final – returned to coach the Romanian ahead of the 2019 WTA Finals, following on from her success at Wimbledon.
The 29-year-old has endured an injury-hit 2021 and was knocked out of the US Open in the fourth round by Elina Svitolina.
In a post on her official Twitter account, Halep announced: "After six wonderful years working together, Darren Cahill and I have decided that it's time to end our working relationship.
"Thank you D for everything, for making me a better tennis player and a better person."
Halep, the 2018 French Open champion, had dominated the first set against Angelique Kerber, winning 6-1, but came off the court with an apparent calf problem with the scores level at 3-3 in the second.
She received treatment but an emotional Halep was forced to withdraw from the clash, the defending champion handing victory to Kerber.
The Romanian was then helped from the grandstand court, leaving the area with strapping on her leg and a heavy limp, raising doubts about her ability to compete at Roland Garros, where she would likely be one of the favourites.
Earlier Williams made her first appearance on the tour since February, losing the 1,000th match of her glittering career to Nadia Podoroska, who reached the semi-finals in Paris last year.
Podoroska prevailed 7-6 (8-6) 7-5, with 23-time grand slam champion Williams not too downhearted by a defeat to an accomplished clay-court player.
"It's tough to have a first match on clay. It was definitely kind of good to go the distance and to try to be out there, but clearly I can do legions better," Williams said.
"She has a good game, for sure, obviously. She's very consistent. Overall, it was good for me to play such a clay-court player on clay today, but it's a little frustrating. But it's all right. It is what it is.
"I have been training for months, but it feels definitely different on clay to make that last adjustment. Just filling out the game, finding the rhythm. Even sliding and confidence with that, with movement. That's always like a little struggle in the first two matches, and then I'm raring to go."
Osaka has yet to find her footing on clay and the reigning Australian Open champion came up short in the second round as she suffered a surprise loss to Jessica Pegula.
Pegula said after her 7-6 (7-2) 6-2 win: "I know she doesn't love clay, so I just tried to be the clay court player out there today, and it worked just good enough."
World number one Ashleigh Barty had no such difficulties in her 6-4 6-1 over Yaroslava Shvedova, but fourth seed Sofia Kenin was a shock loser to Barbora Krejcikova.
Petra Kvitova was beaten in three sets by Vera Zvonareva but Madrid champion Aryna Sabalenka, last year's French Open champion Iga Swiatek, Elina Svitolina, Karolina Pliskova and Garbine Muguruza all progressed while teenager Coco Gauff knocked out 17th seed Maria Sakkari.
