Unseeded American Lilia Osterloh stunned Wimbledon finalist Marion Bartoli 5-7, 6-4, 6-3 in the second round of the Stanford Classic on Wednesday.
The second seeded Frenchwoman's form dipped after receiving treatment for a right quadriceps problem midway through the first set, the injury leaving her moving slowly and without confidence.
While the 22-year-old Bartoli tried to keep exchanges short by ripping balls toward the lines for outright winners, she had little power on her serve and lost nearly every extended rally.
"I'm not completely healthy and my body hasn't recovered from the past two months and it's showing signs of overtiredness," Bartoli told reporters after she left the court in tears.
"I had no energy and felt so sick and it cost me in the match. I'm not pushing hard enough and my serve isn't working at all. But she took all her chances and she can play very well."
Bartoli's cause was hampered further by a bout of stomach flu that she picked up on her arrival in Stanford six days ago.
"When you are not 100 percent, but you are still trying, it's very frustrating," said Bartoli, who has played 31 tournaments over the past year.
"I just hate to lose and losing this way is not a good feeling."
ENTHUSIASTIC FANS
The 29-year-old Osterloh, a former NCAA champion for Stanford University, felt at home on the centre court in front of an enthusiastic group of fans.
Despite losing her way in the first set, she adeptly mixed up her attack in the final two sets, retrieving brilliantly and attacking the net whenever she got a short ball.
After Bartoli erred on a forehand to hand her opponent a break to 5-3 in the third set, Osterloh won the match by booming an ace into the corner.
"Its a great feeling to beat someone who beat the number one and number three players in the world at Wimbledon. It helps my confidence a lot," the world number 106 said.
Osterloh will face eighth seed Sybille Bammer in the quarter-finals after the Austrian overcame American Meilin Tu 6-4 6-2.
Third seed Daniela Hantuchova also advanced with a 6-4, 6-2 victory over China's Peng Shuai.
"It was a great match and she was a tough opponent," the Slovak said. "She played very well and I kept fighting and won all the tough points."
Seventh seed Katarina Srebotnik of Slovenia booked her place in the final eight with a 6-0, 7-6 triumph over Russia's Maria Kirilenko.