Elise Mertens came from behind to clinch a semi-final spot at the Dubai Tennis Championships but Coco Gauff bowed out on Thursday.

Mertens said she "just didn't want to let go" after coming back from the brink to end Jessica Pegula's fine run with a 5-7 7-5 6-0 victory.

The Belgian looked to be heading out when she faced a 5-2 deficit in the second set, yet saved three match points to force a decider.

"I kept on fighting and that was the spirit today," said world number 18 Mertens.

Former world number one Muguruza is Mertens' semi-final opponent, the Spaniard having come from a set down to beat third seed Aryna Sabalenka 3-6 6-3 6-2.

Muguruza lost out to Petra Kvitova in the final of the Qatar Open last week, but looks on top form as she closes in on another championship match.

Sabalenka fell foul of Muguruza in Doha, and the Belarusian was unable to gain revenge as the ninth seed earned a 16th win of the season.

There was to be no place in the last four for 16-year-old Gauff, who came up short against world number 54 Jil Teichmann.

Gauff, who defeated Teichmann twice in Australia in February, was broken to go 5-3 down in the first set and could not respond.

A second break in Teichmann's favour put Gauff on the back foot in the second, and the Swiss then saved three break points to nose into a 3-1 lead.

The contest was settled on Gauff's serve – Teichmann taking the match at the second time of asking to tee up a last-four clash with Barbora Krejcikova, who beat Anastasia Potapova 6-0 6-2 in a little over an hour.

Krejcikova was beaten in the doubles final last year, and has claimed her place in the singles semi-finals of a WTA 1000 event for the first time.

Victoria Azarenka booked her spot in the second round of the Qatar Open with a 6-2 6-3 win over Svetlana Kuznetsova on Monday. 

It was the first time the pair had faced off since the final of the 2016 Miami Open and Azarenka, who won back-to-back titles in Doha in 2012 and 2013, struck first, eventually claiming the opening set in 44 minutes. 

She needed six match points to see off Kuznetsova in the second set to tee up a last-16 meeting with either Laura Siegemund or Elena Rybakina. 

There was also a win for former world number one Garbine Muguruza, who defeated Veronika Kudermetova 6-2 7-6 (7-4).

"Veronika was a very tough opponent," Muguruza told a media conference. "[I'm] just happy with this win. We played a year ago, and I remember it was a tough match. I'm happy to have closed it in two sets."

Fifth seed Kiki Bertens, meanwhile, was dumped out by a rampant Jelena Ostapenko in straight sets. 

The 2017 French Open champion hit 26 winners on her way to an emphatic 6-0 6-2 win against her Dutch opponent, who was playing her first match in five months following Achilles surgery. 

Anett Kontaveit overcame number seven seed beaten Australian Open finalist Jennifer Brady 6-1 6-2 to set up a second-round clash with Angelique Kerber, who beat Cagla Buyukakcay 6-4 6-2.

In the day's other match, Maria Sakkari beat Mayar Sherif 6-0 6-3. 

Meanwhile, teenage qualifier Clara Tauson shocked number one seed Ekaterina Alexandrova 6-3 6-4 in the first round of the Lyon Open. 

There were also wins for Arantxa Rus, Nina Stojanovic, Timea Babos, Greet Minnen and Fiona Ferro in France. 

Ash Barty crashed out of the Adelaide International at her first hurdle after going down in straight sets to Danielle Collins.

It took only 65 minutes for American Collins to defeat the defending champion and world number one in a 6-3 6-4 triumph.

Barty, who had a bye in the first round, did have opportunities, breaking in the first game of the match and building a 4-1 lead in the second set.

But on both occasions Collins stormed back, claiming her first win in four attempts against home favourite Barty, who had beaten her in the semi-finals of last year's tournament.

Coco Gauff continued her fine run to reach a first WTA quarter-final since August 2020 with an impressive comeback win over sixth seed Petra Martic.

The American triumphed 5-7 6-3 6-4 in two hours and 12 minutes to book a clash with compatriot Shelby Rogers at the WTA 500 event.

Gauff did not relinquish serve once after losing the first set and did not even offer up a break-point chance in the decider, which she sealed with a pivotal early break secured with a passing winner.

"I go into every tournament thinking that I can win," said the 16-year-old.

"Obviously that's impossible, to have a perfect record, but I go into every tournament believing that I can win. 

"I try not to think far ahead in the match, at least I just try to focus on my next match, just because you have to get through that before you can get to the finals, and just take it one match at a time."

Second seed Belinda Bencic will face Storm Sanders in the quarter-finals after a 6-1 6-3 win over Misaki Doi.

In a match where she was not quite as dominant as the scoreline suggests, Bencic had to fend off 12 break points but was successful in saving 10 of them.

Number four seed Elise Mertens withdrew before her match with a shoulder injury, so Anastasija Sevastova beat lucky loser Christina McHale to set up a last-eight tie with Jil Teichmann.

Teen stars Iga Swiatek and Coco Gauff earned Adelaide International victories on Tuesday, but third seed Johanna Konta is out.

French Open champion Swiatek secured a 6-3 6-4 triumph over Madison Brengle to set up a last-16 tie against home favourite Maddison Inglis, who beat veteran Sam Stosur in a three-set epic.

"Inside I'm the same person, but I realise that everybody's kind of treating me differently," Swiatek said after the win, as she reflected on the attention she now receives as a grand slam champion.

"So it's something you have to adjust to, and also keep yourself down to earth and the same as you were.

"We did a great job last year and hopefully this season will also be successful. 

"Tennis-wise it also changes - I’m not an underdog anymore, so I start my matches with a different attitude, and that’s the kind of thing you have to learn to do.

"Madison is the kind of player who uses her opponent's power, so I had to stay cool. 

"I tried to calm myself down, stay low in the legs, and sometimes play longer rallies to get in the rhythm, because the first round is always tough at any tournament."

Gauff also progressed, saving eight of the 11 break points she faced to defeat Jasmine Paolini 6-4 6-7 (4-7) 6-2 in a match lasting over two-and-a-half hours.

A tough test against Petra Matric is up next for Gauff in the last 16 after the Croatian sixth seed recovered from a set down to win against qualifier Ludmilla Samsonova.

Gauff's fellow American Shelby Rogers is the first player to reach the quarter-finals.

Konta had a bye in the opening round and had no answers for Rogers as she played her first match of the WTA 500 tournament.

Rogers won 6-2 6-2 in just 76 minutes, never dropping serve in the contest.

Danielle Rose Collins will face world number one Ash Barty after beating Zheng Saisai at the WTA Adelaide International.

The American wrapped up a 7-6 (7-5) 6-1 victory and will now face top seed Barty, who will still be smarting from her surprise quarter-final exit at the Australian Open.

Barty is the reigning champion in Adelaide and will be seeking to put that loss to Karolina Muchova firmly behind her.

Collins' compatriot Shelby Rogers also prevailed, beating Veronika Kudermetova 6-3 7-6 (7-4) to set up a clash with third seed Johanna Konta.

Anastasija Sevastova overcame Caroline Garcia 6-2 6-4, while eighth seed Wang Qiang was a 6-4 6-3 winner over teenager Olivia Gadecki.

Serena Williams has withdrawn from her semi-final at the Yarra Valley Classic due to an injury to her right shoulder.

As a result, home favourite Ash Barty will progress to the final of the Australian Open warm-up event at Melbourne Park.

Williams, who is chasing an eighth Australian Open crown and an historic 24th grand slam singles success, had been in impressive form this week, demolishing Tsvetana Pironkova in straight sets.

Friday's quarter-final against American Danielle Collins proved a harder slog, as she was forced into a match tiebreak to win 6-2 4-6 10-6.

An anticipated showdown against Barty will now not come to pass, with the world number one to pursue a ninth career title against either Garbine Muguruza or Marketa Vondrousova.

"I think the rust is always there for everyone the first few matches of the season. But without a doubt, I felt better and better each match," Barty told reporters after being taken the distance by Shelby Rodgers.

"Each match has been very different, different challenges, different things I've had to overcome, which is the best thing, to be able to work through those and give myself another chance to play a little bit better the next day, focus on some new challenges for the next day."

Williams is due to face Germany's Laura Siegemund in the first round of the Australian Open on Monday.

Serena Williams roared into the quarter-finals of the Yarra Valley Classic with a 6-1 6-4 victory over Tsvetana Pironkova on a star-studded day of action in Australia.

The 23-time grand slam champion was in fine form at Melbourne Park, where the biggest names in women's tennis were competing across multiple tournaments, as she sealed a straight-sets triumph to continue her preparations for a shot at an eighth Australian Open title.

In doing so the 39-year-old extended her head-to-head record over the Bulgarian to 6-0 and was delighted to get the job done.

"It's definitely nice to get another win," the American said during her on-court interview.

"She's clearly a great player, so it wasn't easy, but it was good to come through."

Speaking at her media conference, Williams added: "Last time it was an incredible three-set match, so today I was like, 'All right, let's really try and focus and learn to do better than last time.'"

World number one Ash Barty had a less straightforward path to the last eight, requiring three sets to see off Marie Bouzkova.

The home hope ultimately prevailed 6-0 4-6 6-3, with second seed and reigning Australian Open champion Sofia Kenin also taken the distance in a 5-7 7-5 6-2 win over Jessica Pegula, as Nadia Podoroska dug deep to down Petra Kvitova 5-7 6-1 7-6 (9-7).

With Barty now set to face Shelby Rogers, Danielle Collins awaits Williams having impressively defeated third seed Karolina Pliskova 7-6 (7-5) 7-6 (7-3), while Garbine Muguruza overcame Anastasia Pavlyuchenkova 6-1 6-2 to set up a meeting with Kenin.

In the Gippsland Trophy, the top three seeds all dodged upsets, with two-time grand slam winner Simona Halep, 2019 Australian Open victor Naomi Osaka and 2018 Tour Finals champion Elina Svitolina safely navigating through the last 16.

Fifth-seeded Briton Johanna Konta failed to convert two match points as she succumbed to a 4-6 7-6 (12-10) 7-6 (7-4) defeat to Irina-Camelia Begu.

Meanwhile, at the Grampians Trophy there were wins for Anett Kontaveit, Jennifer Brady and Angelique Kerber.

World number one Ash Barty won her first WTA Tour match in almost a year at the Yarra Valley Classic, while Naomi Osaka got her Gippsland Trophy campaign going with a victory.

Barty sat out the vast majority of the 2020 season due to the coronavirus pandemic but marked her comeback with a 6-3 6-3 success over Ana Bogdan at Melbourne Park.

The Australian will face Marie Bouzkova next after the Czech defeated Aliona Bolsova on Tuesday.

Garbine Muguruza raced past Alison Van Uytvanck 6-2 6-0 and she was joined in the third round by Petra Kvitova, who battled to beat Venus Williams 7-6 (8-6) 7-5.

Reigning Australian Open champion Sofia Kenin, who was 7-5 up when Camila Giorgi retired from their match, is also through.

In her first match since winning the US Open in September, Osaka cruised to a 6-2 6-2 success against Alize Cornet and will meet Katie Boulter after the Briton came from a set down to beat Coco Gauff 3-6 7-5 6-2.

Iga Swiatek was also making her first appearance since winning a major title – the French Open last October – and after a slow start won 10 of the final 11 games to beat Kaja Juvan 2-6 6-2 6-1.

Aryna Sabalenka ended 2020 with back-to-back titles in Ostrava and Linz, before kicking off the new year with another trophy in Abu Dhabi. However, her run of 15 straight victories ended with a 1-6 6-2 1-6 loss to Kaia Kanepi.

Bianca Andreescu was due to gear up for the Australian Open by contesting the Grampians Trophy this week but withdrew from the competition to focus on training instead.