Garbine Muguruza's preparations for next week's Australian Open were thrown into disarray when the world number three pulled out of the Sydney International with a thigh injury on Wednesday.
Hours after battling past Kiki Bertens 6-3, 7-6(6) to advance to the quarter-finals, the 24-year-old Spanish top seed withdrew with a right thigh injury.
The Wimbledon champion accepted a wildcard into the tournament after retiring with leg cramps during her opening match at the Brisbane International last week and called for a medical time-out after the third game against Bertens.
Muguruza, however, returned to the Ken Rosewall Arena in full flow, breaking her Dutch opponent twice to surge 5-1 clear, apparently allaying fears of another injury setback ahead of next week's Australian Open.
Bertens, who had won all three previous meetings against Muguruza in straight sets, recovered one of the breaks to halt her opponent's march but the twice Grand Slam winner regrouped to close out the opening set.
Muguruza broke at the start of the second set against the 26-year-old world number 32 but struggled to impose herself during the key moments, only converting three of 13 break-point opportunities and dropping her serve in the eighth game.
She rallied, however, to seal victory on her fourth match point in the tiebreak when Mertens hit a return long.
"Since the start I felt a little bit my adductor," Muguruza said after registering her first win of the season.
"I already felt it in Brisbane, so I thought I was going to be much better, but it came back.
"You always have to adapt... Sometimes you don't need to have a pain. You just want to play a certain style of game or some tactic."
"I figured out that being a little bit more aggressive, shorter points, that might help me and might help me also for my game. So it's like a winning situation."
Agnieszka Radwanska, the 2013 champion and last year's finalist in Sydney, kept her preparations for the year's first grand slam on course with a 7-6(4), 6-0 win over teenage American qualifier CiCi Bellis.
The Polish 28-year-old will meet Camila Giorgi for a place in the last four after the Italian qualifier ousted twice Wimbledon and former Sydney champion Petra Kvitova 7-6(7), 6-2.
Giorgi, belying her ranking of 100th in the world, beat US Open champion Sloane Stephens in the first round and added another grand slam champion to her list of victims in just under two hours when Kvitova blasted a forehand wide.
In the men's draw, Italian veteran Paolo Lorenzi ousted top seed Albert Ramos-Vinolas of Spain in the second round.
Ramos-Vinolas needed treatment for a leg problem early in the second set and fell 6-3, 7-5 to the 36-year-old Lorenzi, who is ranked 45 in the world.
Del Potro too strong for Shapovalov in Auckland
Argentine Juan Martin del Potro made a confident start to his preparations for the Australian Open with a 6-2, 6-4 win over Denis Shapovalov at the Auckland Classic on Wednesday.
Former US Open champion Del Potro hit eight aces to breeze past the 18-year-old Canadian, who hit 20 winners but committed 40 unforced errors.
The 29-year-old second seed, who won the tournament in 2009 in his last appearance, will now be the favourite after top-seeded American Jack Sock was bundled out by unheralded German Peter Gojowczyk.
Del Potro will next face Russian Karen Khachanov, who beat sixth-seeded Uruguayan Pablo Cuevas 6-2, 7-6(4), in the last eight.
"I was surprised by the match today," said Del Potro, who had lost to Shapovalov in Montreal last August. "I didn't expect to play at this level in the first match of the year."
"But I've been working hard to play this tennis and hopefully I can still improve in the next matches."
"If he has a good day, he is very dangerous as a lefty, he serves well and volleys. Denis played better than me in the Montreal match, but today I broke him three times to love and that was the key."
Defending champion Sock lost 6-3, 6-3 in only 61 minutes to his German opponent, who is ranked 65th in the world.
The Australian Open, the first Grand Slam of the year, runs from Jan. 15-28.
Bopanna and Roger-Vasselin reach semis with two wins in a row
Rohan Bopanna began his partnership with Edouard Roger-Vasselin with consecutive straight sets win in a single day to reach the semi-finals of the Sydney International.
Bopanna and Roger-Vasselin resumed their first match against Ivan Dodig and Fernando Verdasco, leading 4-3 with a break up, and went on to beat them 6-4, 6-4 in 68 minutes.
In the gruelling quarter-finals, the fourth seeds vanquished Fabrice Martin and Nicholas Monroe 7-6(5), 7-6(5) after battling hard for one hour and 34 minutes.
They will next take on top seeded pair of Lukasz Kubot and Marcelo Melo.
Bopanna had played with Pablo Cuevas in the previous season. In the season-opening Tata Open Maharashtra, the tall Coorgi paired with Jeevan Nedunchezhiyan but failed to defend their title.
They lost in the quarter-finals to French team of Gilles Simon and Pierre-Hugues Herbert.
Sabalenka upsets Zhang in Hobart, Watson advances
Belarus teen Aryna Sabalenka blasted top seed Zhang Shuai out of the Hobart International on Wednesday while former champion Heather Watson had a smooth ride to the quarter-finals with a 6-2, 6-2 win over wildcard Jaimee Fourlis.
Big-hitting Sabalenka had felled former world number five Eugenie Bouchard in the first round and the Belarusian's aggressive strategy paid dividends again as she triumphed 7-6(3), 6-4.
"It was a really good fight between us," Sabalenka told reporters after firing 37 winners and committing 39 unforced errors.
"This was a match that I really enjoyed. She fights for every point, and it’s always so special when you feel this fight on court."
Broken early, Sabalenka won four games in a row en route to a 5-2 lead in the first set when Zhang staged a comeback to level it at 5-5 before the Belarusian clinched the set.
Sabalenka was more in control in the second set when she claimed a decisive break at 4-3 before serving out to set up a quarter-final clash with fifth seed Lesia Tsurenko of Ukraine.
"It's always special when you beat two good players in a row in the same tournament," Sabalenka said.
"It gives me more confidence. You finish the match and you really just think, 'When is the next one? I want to go back on court, I want to beat someone.'"
Briton Watson had little difficulty in beating Australian Fourlis, a victory which moves the 2015 champion into a quarter-final against Donna Vekic, and Alison Riske also advanced after Belgian qualifier Kirsten Flipkens pulled out with a right shoulder injury.