American 18-year-old Coco Gauff found the poise to stall a fierce fightback from four-time major winner Naomi Osaka on Thursday, defeating the former world number one 6-4 6-4 to reach the Silicon Valley Classic quarter-finals.
It was the fourth meeting between Japan's Osaka and Gauff, who finished runner-up in her maiden Grand Slam final at this year's French Open, with the American levelling the head-to-head record at 2-2.
Gauff did not allow a single opportunity on her serve in the opening set, with a sole break of Osaka's delivery enough for the American to take it.
She dominated the twice Australian and U.S. Open champion in the second set to go 5-1, 40-0 up before the 24-year-old Osaka turned things around.
Osaka saved four match points to hold serve and then broke Gauff, who committed two double faults, on her first opportunity in the eighth game.
Osaka saved three more match points for her next service hold to narrow the deficit to 5-4 before Gauff converted her eighth match point with another big serve.
"Even though I was up 5-1, 40-0 I knew I didn't have the match in the bag yet and she proved that," Gauff said.
"When you play Naomi she can play lights out tennis. I wasn't too mad. I was just like, eventually it'll calm down and if it doesn't calm down then it's probably not my day.
"The most proud was how I responded mentally. I could have easily folded in those couple of double-faults and match points that I lost. But I kept telling myself, 'another point, another chance,' and then the music kind of helped on the bench. I was trying to sing along to the song to get me out of the match."
Next up for Gauff, who is third behind top-ranked Iga Swiatek and Wimbledon runner-up Ons Jabeur in the Race to the WTA Finals leaderboard, will be second-seeded Spaniard Paula Badosa.
The WTA 500 tournament in San Jose, California lost its top seed on Thursday when world number three Maria Sakkari was beaten 6-1 6-3 by American Shelby Rogers.
