Japan's Ai Sugiyama stunned second seed Svetlana Kuznetsova 2-6, 6-4, 6-3 in one of several upsets in the third round of the Acura Classic in Carlsbad, California on Thursday.
Russian 18-year-old Anna Chakvetadze upended ninth seed Elena Likhovtseva 6-3, 6-1 and China's Shuai Peng took down number 16 seed Dinara Safina 7-6, 6-0.
Japan's Akiko Morigami, meanwhile, ended Indian qualifier Sania Mirza's run with a 6-2, 4-6, 6-4 victory.
The 30-year-old Sugiyama counterpunched brilliantly against US Open champion Kuznetsova, who was rusty after a month's break since Wimbledon.
Kuznetsova got off to a fast start but in the second set, the Japanese veteran began to wear her down with sharply angled shots.
In the third set, Kuznetsova committed a series of unforced errors to be broken to 4-2 and lost the contest when she flailed at an easy backhand.
"She concentrated during the whole match and I didn't," said Kuznetsova.
"I had too many ups and downs. That month was the longest break I've ever had from tennis and I need some more matches before I get my form back."
After winning her first Grand Slam title at the 2004 US Open and taking the title in Bali the following week, Kuznetsova has lost her stride. She has not won a tournament since then or reached the semi-finals of a Grand Slam.
"I'm not very satisfied with my year," Kuznetsova said. "If you compare it to last year, maybe it's 50 percent as good, but I can't ask that of myself all of the time. Hopefully with more matches, my confidence will come back before the US Open."
A one-time member of the world top 10, Sugiyama's level has also dropped. At the end of last year when her standard began to fall, she thought about retiring.
"I'd lost my energy and my motivation," said Sugiyama, who reached her first quarter-final of the year.
But Sugiyama hired a new hitting partner, Italy's Simone Amorico, and is also travelling full time with a fitness trainer.
"My goal is to get back into the top 10. My fitness is getting better [and] with the new team I'm getting my confidence back," she said. "I don't to end my career not playing at the level I'm capable of."
Sugiyama will face 15-year-old Sesil Karatantcheva, who downed American Mashona Washington 6-1, 6-2.
It is the first time since 1996 that there are no Americans in the last eight and only the second time in the 21-year history of the tournament.