World number one Iga Swiatek commenced her San Diego Open campaign with a hard-fought three-set victory over Chinese lucky loser Zheng Qinwen in rainy conditions on Thursday.

The 2022 French Open and US Open champion triumphed 6-4 4-6 6-1 in two hours and two minutes, bouncing back strongly after losing the second set.

Zheng's power and forehand top spin proved challenging for the Pole, who was solid from the baseline to secure her 11st tour quarter-finals appearance this season.

The Chinese had broken Swiatek to lead 1-0 in the third set, but the top seed responded in trademark style, improving her record in the United States this season to 21-1.

Swiatek will take on eighth seed Coco Gauff in the quarter-finals, with the match-up marking a re-match of this year's French Open final.

Gauff worked her way into the last eight with a 6-4 4-6 6-3 victory over 2019 US Open champion Bianca Andreescu.

The 18-year-old American also had to fight back from a set down in the third set to win in a match that lasted just over two-and-a-half hours.

Third seed Aryna Sabalenka fought back from a lopsided opening set to knock out American Sloane Stephens 1-6 6-3 6-2 in two hours and one minute. The Belarussian will take on Donna Vekic in the next round.

Second seed Paula Badosa eased into the last eight, winning in 53 minutes against American qualifier Louisa Chirico 6-0 6-3. Badosa will face Danielle Collins in the quarter-finals.

Madison Keys triumphed over eighth seed Daria Kasatkina 6-4 6-3 in one hour and 27 minutes to secure a quarter-final date with fourth seed Jessica Pegula.