Belinda Bencic upset world No 3 Coco Gauff on Wednesday to line up a quarterfinal clash with Australian Open champion Madison Keys at Indian Wells.
Switzerland's Bencic triumphed 3-6, 6-3, 6-4 to reach her first WTA 1000 quarterfinal since becoming a mother last year.
Keys also had to rally, downing Croatia's Donna Vekic 4-6, 7-6 (9/7), 6-3 to advance in her first tournament since she captured her maiden major in Melbourne.
Bencic dominated after dropping the first set to avenge a fourth-round loss to Gauff at the Australian Open this year.
The 28-year-old former world No 4 didn't face a break point after the opening set and kept the pressure on Gauff, who struggled with her forehand after her aggressive start to the game.
It was a loose forehand return which handed Bencic the crucial second break point at 4-4 in the third set and she confidently served out to clinch victory.
Bencic, who gave birth to daughter Bella last April, was unranked when she returned to action in October. But she has gone from strength to strength, claiming her ninth career title in Abu Dhabi last month to rise to 58th in the world this week.
She said her improved fitness was a factor in her ability to best Gauff in three sets this time, when she couldn't in Melbourne in January.
"What changed is that I just kept on working also on the physical side," Bencic said.
"I felt like in the last matches in tournaments, I kind of was able to play back to back, some three-set matches, and am fresher every time.
"The body is holding up much better, almost like it was before."
Keys unleashed a barrage of winners to vanquish Vekic, but after dropping the opening set and failing to convert a set point in the 10th game of the second the American had to gut out a tense second-set tiebreaker in which Vekic moved within two points of victory when she led 5-3.
Keys broke Vekic in the opening game of the third to give herself some breathing room and broke her again to lead 5-2. Vekic managed to pull back one break but Keys gave herself double match point with a blistering backhand service return in the next game and sealed the victory on her second opportunity.
Women's world No 1 Aryna Sabalenka of Belarus eased into the quarterfinals with a 6-1, 6-2 victory over British lucky loser Sonay Kartal.
Sabalenka will next face either world No 6 Jasmine Paolini of Italy or Russian Liudmila Samsonova.