Defending champion Caroline Wozniacki came from a set down to beat Sweden's Sofia Arvidsson in the third round of the BNP Paribas Open at Indian Wells.
Dane Wozniacki, the fourth seed, initially struggled with her serve on a cool evening and dropped the opening set 6-3.
But she rediscovered her touch to take the second 7-5 and broke three times in the decider, winning it 6-2 to complete victory in two hours and 37 minutes.
"I knew it was going to be a tough match," said Wozniacki. "I've known Sophia since I was 12 or 13 and we know each other's games so well.
"She just came out from a win in Memphis and I knew she was on fire. I'm so happy with this win."
Wozniacki will play another former world number one, Serb Ana Ivanovic, in the fourth round. Ivanovic was a 6-7 (8/10) 6-3 6-2 winner against Kazakhstan's Ksenia Pervak.
Second seed Maria Sharapova enjoyed an easier route into the fourth round with a straight-sets victory over Romanian Simona Halep.
The Russian prevailed 6-3 6-4 in one hour and 36 minutes, although she had to fend off a second-set fightback from Halep, who won four games in a row from 5-0 down before finally succumbing.
"I played extremely well the first four or five games of that set -almost too good in a way," said Sharapova.
"I was going for a lot and making a lot of shots. Then felt like I almost started going for a little bit too much.
"Instead of being patient I had a few errors that I shouldn't have made. All of a sudden your opponent gains confidence because they have nothing to lose. It's a bad situation but I got it together in the end."
Earlier on Monday, Nadia Petrova upset US Open champion Sam Stosur in their third-round clash.
The Russian, seeded 30, pulled off a 6-1 6-7 (6/8) 7-6 (7/5) win over the sixth-seeded Australian, who has struggled this season following her triumph at Flushing Meadows last September.
Petrova served 15 aces during the match before clinching victory after two hours and 46 minutes on court.
In contrast, seventh seed Marion Bartoli needed only 71 minutes to dispatch South Africa's Chanelle Scheepers 6-2 6-0.
And there were also wins for Maria Kirilenko, Roberta Vinci and Lucie Safarova at Indian Wells on day six of the tournament.
