Mertens, who reached the fourth round at Wimbledon at the start of July, succumbed to a 6-3 2-6 6-2 defeat to Magda Linette – her first loss to the Pole after three straight-set victories in their previous encounters.
The Belgian got just 52 per cent of her first serves in and won 38.5 per cent of points behind her second serve while only converting one of the five break points she earned.
Sixth seed Alison Van Uytvanck retired in the first set of her match against Dalila Jakupovic, while eighth seed Marie Bouzkova overcame Sinja Kraus 6-2 7-6 (7-2).
In Warsaw, fifth seed Garcia by no means had it easy in a 7-5 6-4 success over Doi.
The Frenchwoman committed eight double faults and only got 55 per cent of her first serves in, but by saving six of the eight break points she faced she was able to get the job done in straight sets.
Polish wildcard Maja Chwalinska was a 7-5 6-2 victor over qualifier Rebeka Masarova in only her second Tour-level tournament.
World number 10 Muguruza, who had only won two tour-level matches since the start of March, was tested in a gripping second-set tie-break by Magdalena Frech, eventually triumphing 6-1 7-6 (9-7).
After the victory, Muguruza said she was pleased to have overcome challenging weather conditions to reach the next stage, saying: "I feel like I fought hard against the wind, and I haven't played many matches on grass either.
"I could see today that if I didn't fight as hard as I can, I wasn't going to win."
A minor upset saw 17th seed Alison Riske slip to a 6-7 (3-7) 6-3 7-6 (7-4) loss to Magda Linette, while Yulia Putintseva surged to a 7-5 6-1 win over Sara Sorribes Tormo and seventh seed Barbora Krejcikova saw her contest with Marta Kostyuk halted by darkness at one set apiece.
Ajla Tomljanovic beat Aleksandra Krunic 6-0 4-6 6-3 and Heather Watson went down 7-6 (7-1) 6-4 to Canada's Rebecca Marino, as the Sussex crowds prepared for Serena Williams' eagerly anticipated doubles outing alongside Ons Jabeur on Tuesday.
There was frustration for Muguruza's fellow former Wimbledon champion Angelique Kerber at the Bad Homburg Open.
Kerber's match against Anastasia Gasanova got under way, but play was stopped due to fading light and a damp court after a rain-hit day. Third seed Kerber led 4-2 in the first set of a contest that will conclude on Tuesday.
The only other seed in action in Germany made routine progress, as France's Alize Cornet raced to a 6-2 6-4 win over Anna Kalinskaya.
Raducanu required just 77 minutes to see off the third seed for a 6-2 6-2 win, dominating from the outset to clinch her first final four spot in any tournament since last year's famous US Open triumph.
Having wrapped up the opener with a fierce backhand winner, Raducanu was forced to endure a brief revival from Linette in the second set, before the Pole was troubled by a thigh injury.
Linette lost 14 of the first 17 points on resumption after a medical time out, struggling with her movement as Raducanu saw out the match.
Raducanu has yet to drop a set during her run in Seoul, but will face a stern test in the semi-finals after Ostapenko overcame 17-year-old Victoria Jimenez Kasintseva 6-2 6-1, setting up a first career meeting between the duo.
On the other side of the draw, Ekaterina Alexandrova earned a hard-fought 7-5 7-6 (8-6) win over Lulu Radovcic, but fourth seed Zhu Lin crashed out to Tatjana Maria, losing 6-1 6-1.
In the Pan Pacific Open in Tokyo, meanwhile, Garbine Muguruza fell to a shock 6-4 6-2 quarter-final defeat to Liudmila Samsonova, handing the Russian her 16th win in her last 17 outings.
She will meet Zhang Shuai in the next round, after she became the first Chinese player to make the last four in Tokyo since 2009 by beating Petra Martic 7-5 6-2.
In a battle between the competition's fourth and fifth seeds, meanwhile, Veronika Kudermetova rallied to beat Brazil's Beatriz Haddad Maia 6-7 (4-7) 7-6 (8-6) 6-1.
Sabalenka could not quite hit her peak form against Linette, but the world number two nevertheless got the job done after going the distance.
The Belarusian is hunting a record-equalling third title in Madrid, where she is the reigning champion, though she has not won back-to-back matches at a tournament since winning the Australian Open.
"It's not about being confident," Sabalenka said. "It's about how much you're ready to do to get it. It's about the hard work and to be ready, be ready for the big fights. I feel like confidence is not going to help you in those big matches. It's about staying there and fighting for it."
Data Debrief
Sabalenka (70 per cent, 56-24) is now one of five active players since 2020 to hold a winning percentage of 70 per cent or higher at WTA-1000 events.
Iga Swiatek, Simona Halep, Elena Rybakina and Jessica Pegula are the other players to feature on that list.
Kazakhstan's Rybakina, who is also the reigning Wimbledon Champion and Australian Open finalist, got the better of Badosa for the second time during her current run, also eliminating her from Indian Wells.
The 23-year-old has to be considered one of the favourites to go all the way, having knocked off world number one Iga Swiatek and world number two Aryna Sabalenka to lift the trophy in California.
Rybakina will meet Belgium's Elise Mertens in the fourth round after she beat Croatia's Petra Martic 6-4 6-3.
Meanwhile, the biggest upset of the day was delivered by Russia's Anastasia Potapova, bouncing sixth seed Coco Gauff 6-7 (8-10) 7-5 6-2.
Potapova, who has never won a tournament above the WTA 250 level, will face China's Zheng Qinwen for a spot in the quarter-finals after her three-hour 5-7 7-6 (7-5) 6-3 triumph over Liudmila Samsonova.
Latvia's Jelena Ostapenko, the 24th seed, eliminated Brazilian 13th seed Beatriz Haddad Maia 6-2 4-6 6-3, while 20th seed Magda Linette of Poland knocked out Belarusian 14th seed Victoria Azarenka 7-6 (7-3) 2-6 6-4.
Unseeded American Clare Liu went down 4-6 7-5 6-4 against Italy's Martina Trevisan, but third seed Jessica Pegula will continue to fly the flag for the United States after advancing 6-1 7-6 (7-0) in her all-American showdown with Danielle Collins.
The Kazakhstan representative broke Bogdan five times while saving all eight break points to seal a 6-1 6-1 triumph and reach a first final since her SW19 victory over Ons Jabeur in July.
In the final, Rybakina will face Katerina Siniakova after she overcame Anna-Lena Friedsam.
Siniakova stormed to the first set, but the second was a much closer affair and the world number 82 watched five match points come and go before she finally clinched a 6-1 7-5 success.
At the Chennai Open, Magda Linette will play Linda Fruhvirtova in Sunday's decider after coming through semi-final encounters in differing fashion.
Third seed Linette raced out to a 3-0 lead in the first set over Katie Swan before the Brit called a medical timeout after just 16 minutes of play, eventually retiring with illness.
Fruhvirtova's semi-final was a much more gruelling affair, as the 17-year-old came from a set down to overcome Nadia Podoroska 5-7 6-2 6-4 and reach her first WTA final.
The Czech secured the third singles triumph of her career – and first since prevailing at Bastad in 2017 – as she recovered from losing the opening set to deny Wimbledon champion Rybakina 6-7 (4-7) 7-6 (7-5) 6-4.
Siniakova – also the doubles world number one – continued her rich vein of form, having captured her third grand slam of 2022 alongside Barbara Krejcikova at the US Open last week.
The world number 82 had come through two matches on Saturday to reach the championship match in Portoroz, and was slow out of the blocks as she fell 5-0 behind in the opening set. Although Siniakova fought back to force the tie-break, Rybakina held her nerve by winning four of the last five points to draw first blood.
The Wimbledon champion, who hit 43 winners, appeared to be closing in on victory in her third final of the year as she broke for 4-3 in the second set. However, Siniakova broke back immediately before eventually forcing a decider.
Both players exchanged breaks early on in the third, but Rybakina's increasing unforced error tally (66) eventually culminated in her opponent claiming another crucial one for 5-4, before serving it out to end the drought.
Over at the Chennai Open, teenager Linda Fruhvirtova also enjoyed a comeback victory as she denied Magda Linette 4-6 6-3 6-4 to land her maiden WTA title.
Aged 17 years and 141 days, Fruhvirtova became the youngest winner on the WTA Tour this season, and is set to climb into the world's top 100 for the first time.
In a tight opening set, Linette broke for 5-4 before holding to love to strike first. A solitary break also proved crucial in the second set; the Czech profiting on this occasion to ensure the final went to a decider.
Linette, who won just two out of nine break points, established early control as she surged into a 4-1 lead, but Fruhvirtova came roaring back as she reeled off five straight games to seal a fitting end to her breakthrough week.
The second seed won the first set via a tie-break and came back from a double break down in the second to seal a 7-6 (7-0) 7-5 victory in an hour and 58 minutes.
Linette was just two points away from winning the opener and missed an opportunity to force a decider on set point in the second.
"It definitely was not easy. She was fighting back, she was trying to produce a good level; I was up and down," said Svitolina, who reached the quarter-finals on her return to the WTA Tour at the Internazionali d'Italia last week.
"It's not easy to come from another tournament with different balls, different conditions. I was trying just to fight and, in the end, I was lucky I finished in two sets."
Jil Teichmann awaits Svitolina in the last eight following a 7-6 (7-3) 7-5 win against 2018 champion Anastasia Pavlyuchenkova, while there were also victories for Elena Rybakina and Zhang Shuai.
Mladenovic was no match for Svitolina, who steamrolled the seventh seed in a brutal 6-1 6-0 victory at the WTA 250 tournament on Thursday.
Ukrainian star Svitolina reeled off the last 11 games to set up a semi-final clash with Rebecca Peterson.
Peterson moved through 7-6 (7-4) 2-6 4-1 after Tereza Martincova retired hurt.
French ninth seed Alize Cornet rallied past fifth seed Marketa Vondrousova 2-6 6-4 6-3, while eighth seed Marta Kostyuk won the first set 7-6 (7-4) but trailed Varvara Gracheva 5-3 in the second before the match was suspended due to darkness.
At Tennis in the Land, top seed Daria Kasatkina was upstaged by Magda Linette 6-1 6-2 in the quarter-finals.
Second seed Anett Kontaveit survived 6-3 5-7 7-2 against Katerina Siniakova in Cleveland, while fellow seed Sara Sorribes Tormo also won through to set up a semi-final clash.
After dropping just one game in her quarter-final win over Kristina Mladenovic, Svitolina needed nearly two and a half hours to dispatch Peterson 6-1 6-7 (4-7) 6-3 on Friday.
The top seed will seek her 16th career WTA Tour title against ninth-seeded Alize Cornet, who rallied to defeat Varvara Gracheva 4-6 6-1 6-0 in the other semi.
Gracheva's capitulation after winning the first set was understandable, as earlier in the day she had finished off a quarter-final upset of eighth seed Marta Kostyuk in a match that had been suspended due to darkness Thursday.
Gracheva prevailed 6-7 (4-7) 6-3 6-4 in that contest, with play on Friday beginning late in the second set, but did not have enough to pull out another win.
The final will feature the eighth head-to-head meeting between Svitolina and Cornet, with the Ukrainian holding a 4-3 edge.
At Tennis in the Land, second seed Anett Kontaveit defeated seventh seed Sara Sorribes Tormo 6-4 6-4 in one semi-final.
She will meet Irina-Camelia Begu in the final of the Cleveland tournament after the unseeded Romanian upset sixth seed Magda Linette 7-6 (7-5) 6-2.
Kontaveit won her first WTA title at Guadalajara earlier this year, while Begu will be seeking her fifth Tour title and first since 2017.
The 21-year-old Pole, who won last year's Indian Wells, had match point at 6-0 5-0, before Liu rallied back on serve to avoid the ignominy of a double-bagel defeat, with Swiatek eventually winning 6-0 6-1.
Swiatek was in a dominant mood, winning 57 of 83 points for the match and converting five of nine break points generated. She dropped only 11 points in seven service games.
The three-time grand slam champion is 13-3 on the season, with all 13 wins coming in straight sets. In five of those matches, she has dropped just one game.
Former US Open winner Emma Raducanu moved into the third round with a 7-6 (7-3) 6-2 win over 20th seed and 2023 Australian Open semi-finalist Magda Linette.
Raducanu's win meant she has put together back-to-back victories for the first time since September, setting up a clash with 13th-seed Beatriz Haddad Maia after she beat Katerina Siniakova 5-7 7-6 (7-4) 6-3.
Fourth seed Ons Jabeur fought back from a set down to prevail over Magdalena Frech 4-6 6-4 6-1 in one hour and 44 minutes.
Former US Open champion Bianca Andreescu also overcame an early deficit to win 4-6 6-4 6-3 over Peyton Stearns.
Reigning Wimbledon champion Elena Rybakina got the edge in a tight two-set clash with 2020 Australian Open winner Sofia Kenin, triumphing 7-6 (8-6) 7-6 (7-5) in two hours and 10 minutes. Rybakina fought back from 4-1 down in the second frame.
Fifth seed Caroline Garcia overcame a wobble to win 6-1 6-7 (4-7) 6-4 over Hungary's Dalma Galfi, with 30th seed Leylah Fernandez her next opponent after beating Emma Navarro 6-2 6-4.
Two-time major winner and three-time US Open finalist Victoria Azarenka was the big casualty from the day's play, losing 7-6 (7-1) 6-3 to 2021 Australian Open semi-finalist Karolina Muchova.
Poland trailed after Lorenzo Musetti and Matteo Berrettini secured wins against Daniel Michalski and Hubert Hurkacz respectively, only for Swiatek and Magda Linette to level things with respective singles victories over Martina Trevisan and Lucia Bronzetti.
That sent the Brisbane city final to a decider in the mixed doubles, which Swiatek and Hurkacz won with ease 6-1 6-2 against Musetti and Camilla Rosatello, though there was a pause in the second set to dry rainfall on the court.
Despite defeat, Italy also advanced to the final four as the city finals loser with the best record.
It was more routine in Sydney as the United States eased to a 4-1 win against Great Britain.
Madison Keys came from a set down to beat Katie Swan, while Jessica Pegula eased past Harriet Dart 6-2 6-0 after Cameron Norrie had defeated Taylor Fritz in three sets.
Despite going a set behind, Frances Tiafoe came back to beat Dan Evans 3-6 7-5 6-3 to clinch the win for his country, before Pegula and Fritz added some gloss after beating Dart and Evans in the mixed doubles.
The third city final in Perth included a thriller as Greece's Stefanos Tsitsipas beat Borna Coric 6-0 6-7 (4-7) 7-5. Donna Vekic had no trouble getting Croatia on the board as she eased past Despina Papamichail 6-2 6-0.
Maria Sakkari put Greece 2-1 ahead when she beat Petra Martic 6-3 6-3, but Borna Gojo defeated Stefanos Sakellaridis in straight sets to make it 2-2.
However, Greece secured the win and their place in the last four after the pairing of Sakkari and Tsitsipas saw off Martic and Gojo 7-6 (8-6) 6-4.
Greece will take on Italy in the semi-finals, while Poland face the USA, with both taking place in Sydney on Friday.
Second seed Tomljanovic wrapped up a 6-2 6-1 win over her Spanish opponent in Rabat on Sunday, kick-starting her bid to better her second-placed finish at the 2018 edition of the event.
The world number 44, who was beaten by fellow Australian Ashleigh Barty in the Wimbledon quarter-finals after overcoming Emma Raducanu in London last year, is on course for a potential meeting with Croatian 16-year-old Petra Marcinko, who shocked ninth seed Rebecca Peterson to claim a three-set victory.
The Australian Open girls' champion and junior world number one claimed a superb 6-0 2-6 7-5 win over the Swede for her first career top 100 triumph.
At the Internationaux de Strasbourg, meanwhile, eighth seed Linette saw off Varvara Gracheva 6-3 6-0 to reach the round-of-16, setting up a potential tie with Heather Watson.
Elsewhere, Harmony Tan downed Australia's Samantha Stosur 6-2 6-1 to set up a potential round-of-16 meeting with Shuai Zhang, who faces Belgium's Maryna Zanevska on Monday.
Second seed Tomljanovic was a 7-5 6-2 victor against Anna Karolina Schmiedlova, reeling off the last four games in succession to advance in Morocco.
Fellow Australian Astra Sharma joined her in the last eight by overcoming wildcard Petra Marcinko 5-7 6-3 6-3.
Kristina Mladenovic had to save a match point after passing up two herself to finally get over the line with a 6-3 3-6 7-6 (9-7) success - her first singles win of the year - over lucky loser Tessah Andrianjafitrimo in a first-round clash.
At the Internationaux de Strasbourg, third seed Cirstea stormed to a one-set advantage against Ekaterina Makarova but soon fell away and succumbed to a 1-6 6-3 6-2 loss.
Magda Linette breezed past Heather Watson 6-1 6-1 to reach the quarter-finals, where she was joined by Maryna Zanevska after the Belgian's 4-6 7-5 6-1 comeback win against Harmony Tan.
Top seed Karolina Pliskova awaits American Bernarda Pera who defeated Elena-Gabriela Ruse in straight sets, while Anna-Lena Friedsam will take on Elise Mertens after rallying to beat Daria Saville 4-6 6-3 7-6 (7-5).
Gracheva, ranked 88th in the world, needed two hours and 31 minutes to get past Linette 6-3 4-6 6-4 in Austin.
The Pole had led 2-0 in the third set before Gracheva hit back with three breaks in the final frame to run away with a shock victory.
Linette's early exit followed that of second seed Zhang Shuai who withdrew from the tournament on Tuesday due to illness.
Fifth seed Sloane Stephens had no such troubles, progressing into the second round with a 6-3 6-3 victory over Taylor Townsend.
Fourth seed Danielle Collins won 6-7 (7-9) 6-2 6-4 over Magdalena Frech in two hours and 32 minutes, while third seed Anastasia Potapova eased past Elizabeth Mandlik 6-2 6-0.
Top seed Caroline Garcia cruised into the second round at the Monterrey Open in Mexico on Tuesday, winning 6-3 6-4 over Slovenia's Kaja Juvan.
Third seed Donna Vekic survived an early scare to win by virtue of a walkover at 2-6 5-0 against Lesia Tsurenko who retired for the second straight tournament.
Fourth seed Elise Mertens was too strong for Diana Shnaider 6-0 6-4, while seeds Mayar Sherif and Elisabetta Cocciaretto also progressed.
Sixth seed Katerina Siniakova, who made last week's Mexican Open semi-finals, was eliminated after a controversial call on match point in a third-set tie-break, losing to Kamilla Rakhimova 7-6 (8-6) 2-6 7-6 (7-5).
Last week's Mexican Open champion Camila Giorgi was knocked out 6-4 7-5 by Romanian qualifier Elena-Gabriela Ruse.
The Belgian already has collected a pair of grand slam doubles titles this year at the Australian Open and Wimbledon and is seeking her seventh career singles crown.
She will need to get past fourth seed Daria Kasatkina to have a chance.
The Russian outlasted Magda Linette 6-4 3-6 6-4 in her quarter-final and is seeking her third title of 2021 after a three-year drought between wins.
The other semi-final will match seventh seed Danielle Collins against qualifier Ana Konjuh.
Collins pulled out a tight 7-6 (7-5) 7-6 (7-4) upset of second seed Elena Rybakina to reach the semis in San Jose for the second time.
The American won her eighth consecutive match after taking the title at Palermo two weeks ago.
She will meet the only unseeded player remaining, Konjuh, who won the final 10 games of the match to close out Zhang Shuai 3-6 6-2 6-0.
Zhang was never in danger against her unseeded opponent, easing to a 6-2 7-6 (7-4) win to set up a last-eight clash with Tereza Martincova.
Martincova brushed aside number eight seed Magda Linette 6-2 7-6 (7-3) in their round-of-16 meeting on Wednesday.
Elsewhere, Viktorija Golubic beat Heather Watson 7-5 6-2, while the match between number three seed Camila Giorgi and Harriet Dart was suspended at the start of the third set after the Briton had made it one set each.
Meanwhile, rain caused havoc at the Rosmalen Grass Court Championships, with top seed Aryna Sabalenka's match against Arianne Hartono the only one to partially take place.
The match was called off for the day after Sabalenka had claimed the first set 6-2.
