Foot injuries cost the St. Petersburg Open three seeded players on Wednesday, including No. 1 Nikolay Davydenko and No. 5 Jarkko Nieminen.
Former champion and seventh-seeded Mikhail Youzhny also withdrew after spraining his Achilles tendon in a doubles match with Nenad Zimonjic.
Top seed Davydenko quit his second-round match against South Africa's Wesley Moodie with a foot injury while leading 6-2, 3-3.
Just 30 minutes earlier eighth-seeded Youzhny pulled out of the US$1 million ATP Master Series tournament after spraining his right ankle in a doubles match. Swede Thomas Johansson received a walkover into the singles quarter-finals as a result.
"I hurt my right foot in the seventh game of the first set when I ran for a short ball," Davydenko told a news conference.
"I probably could have continued but I wasn't sure how I would feel tomorrow and I just didn't want to take any chances with a Masters tournament in Paris coming up next week and the [season-ending] Masters Cup in Shanghai to follow," he said.
Youzhny's injury appears to be more serious.
The world No. 23 was taken to a local hospital for X-rays.
"Medical tests will show if Youzhny would be able to participate in any future tournaments for the rest of the year," ATP officials said in a statement.
Nieminen retired with a foot injury while trailing Potito Starace of Italy 7-5, 3-2.
"It's a pity I had to quit the event this way," Nieminen said. "The pain was growing and I could not keep on playing."
Defending champion Nadia Petrova dispatched Shahar Peer of Israel 6-1, 6-1 in the second round of the Generali Ladies on Wednesday.
Given a first-round bye, second-seeded Petrova was expected to have a tough match with Peer, a three-time champion on the WTA Tour this year. But the Russian fired seven aces and didn't allow a break point while breaking Peer five times.
Also through to the quarter-finals was fourth-seeded Nicole Vaidisova of the Czech Republic after beating Mara Santangelo of Italy 7-6 (4), 6-0.
In first-round action, the fifth-seeded Ana Ivanovic of Serbia defeated Michaella Krajicek of the Netherlands 6-1, 7-6 (2), and No. 6 Jelena Jankovic of Serbia downed wild card Sybille Bammer of Austria 6-1, 6-7 (3), 6-3.
Qualifier Eleni Daniilidou of Greece beat Chanda Rubin of the US 6-1, 6-2 to set up a second-round match against top-seeded Maria Sharapova.
Former champs David Nalbandian and Fernando Gonzalez stayed in the chase for next month's Masters Cup places as both advanced into the second round of the Swiss Indoors on Wednesday.
Second seeded Argentine Nalbandian, who lifted the title in 2002 before losing the next two finals, was ruthless in his 6-2, 6-4 demolition of Croat Roko Karanusic.
Holder Gonzalez of Chile had to come through a slugfest to get past an Italian with an upset in mind, taking 2 hours, 19 minutes to subdue his feisty challenger Simone Bolelli 7-6 (7/3), 6-7 (2/7), 7-6 (7/3).