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MOSCOW, Russia -- U.S. Open champion Maria Sharapova pulled out of the Kremlin Cup on Friday because of a right foot strain.
Top seed Amelie Mauresmo also made her exit, toppled by eighth seeded Czech Nicole Vaidisova.
Vaidisova rallied to win 1-6 7-5 7-6.
The second-seeded Sharapova beat Eketerina Bychkova in straight sets on Wednesday and should have faced Anna Chakvetadze in the quarterfinals on Friday.
"Unfortunately, I will not be able to play today," Sharapova said.
"I felt pain in my right hip back in the hotel after my first match here.
" I couldn't practice and even walk yesterday. I have to pull out. I'm really disappointed but I have no choice."
On her debut at a home event last season, the then top-ranked Sharapova lost to Dinara Safina in the quarterfinals.
Chakvetadze was joined in the semifinals by fifth seed Nadia Petrova, who beat fellow Russian Vera Zvonareva 7-5 6-1.
In the men's tournament top seed Nikolay Davydenko will face veteran Frenchman Fabrice Santoro in the semifinals.
Davydenko, chasing a place in the season-ending Masters Cup in Shanghai next month, beat Max Mirnyi of Belarus 6-3 7-5 to edge closer to his sixth final of the year.
The Russian stands fifth in the ATP Champions Race and victory in Moscow would go a long way to securing a place in the eight-man finale.
Santoro, the 33-year-old seventh seed, booked his place in the last four with a 6-3 7-5 defeat of Germany's Philipp Kohlschreiber.
Sharapova said she had been unable to walk because of the injury.