The Kremlin Cup lost its two top-seeded women on Friday when Amelie Mauresmo was upset by Czech teenager Nicole Vaidisova and second-seeded Maria Sharapova pulled out with a foot injury.
Vaidisova, seeded eighth, rallied to win 1-6, 7-5, 7-6 (3) and upset top-ranked Mauresmo for the second consecutive time. She also downed Mauresmo in straight sets in the fourth round of the French Open.
On Saturday, Vaidisova will play fifth-seeded Russian Nadia Petrova, who advanced to her second straight semifinal with a 7-5, 6-1 win over compatriot Vera Zvonareva.
Petrova won the Porsche Grand Prix in Stuttgart, Germany last weekend - her fifth title this season - and has not dropped a set in four matches here.
Mauresmo, looking for her fifth title this season, had little trouble against 17-year-old Vaidisova in the first set, winning six consecutive games.
She led 5-2 in the second set but Vaidisova held on and saved a match point to level 5-5 before breaking her opponent to take the match into a third set.
In the third-set tiebreaker, Mauresmo saved a match point before Vaidisova rushed to return a drop shot with the court wide open. U.S. Open champion Maria Sharapova pulled out with a foot injury ahead of her quarterfinal match against fellow Russian teenager Anna Chakvetadze.
"Unfortunately, I will not be able to play today (Friday)," Sharapova said. "I couldn't practice and even walk yesterday. I have to pull out. I'm really disappointed but I have no choice."
Chakvetadze, who won her first title two weeks ago at the Guangzhou Open, China, will take on the winner of the match between Elena Dementieva and Patty Schnyder.
In the men's action, top-seeded Nikolay Davydenko defeated Max Mirnyi 6-3, 7-5 on Friday and will play Fabrice Santoro in the semifinals of the Kremlin Cup.
Davydenko, ranked fifth, saved three breakpoints in the eleventh game of the second set.
"Mirnyi is a very inconvenient opponent for me. He plays serve-and-volley but is not that strong from the baseline," Davydenko said. "Santoro plays classical tennis and is a very convenient opponent for me. I hope I will win tomorrow (Saturday)." Seventh-seeded Santoro made it to the final four with a 6-3, 7-5 win over Philipp Kohlschreiber of Germany.