Top seed Nikolay Davydenko pulled out of his second-round match at the St Petersburg Open with a foot injury when leading Wesley Moodie 6-2 3-3.
Davydenko is currently fifth in the world rankings
The injury could threaten the Russian's hopes of competing in the season-ending Masters Cup.
It is also bad news for Russia ahead of the Davis Cup final, especially as Mikhael Youzhny is also out injured.
Youzhny tore ligaments in his right ankle in a doubles match at the St Petersburg Open on Wednesday.
The Russian was seen on crutches at St Petersburg
airport leaving for Moscow on Thursday morning and has no chance of being fit for the Davis Cup final in Moscow against Argentina on 1-3 December.
"Youzhny damaged his ligaments, so he'll miss at least six
to eight weeks, possibly more," said an ATP spokesman.
With former top 20 player Igor Andreev, who won last year's
Kremlin Cup at the Olympic Sports Complex which is hosting the Davis Cup final, still recovering from knee surgery, Russia are struggling for fit players.
To cap a miserable day for Russian fans, sixth seed Dmitry Tursunov was upset 4-6 6-3 6-4 by Max Mirnyi of Belarus in the first round.
On a day of enforced withdrawals, fifth seed Jarkko Nieminen of Finland also retired hurt from his second-round match against Potito Starace.
The Italian was leading 7-5 3-2 when Nieminen pulled out.
German Tommy Haas was one of the few seeds to enjoy success on Wednesday.
But the fourth seed needed three sets to see off Nenad Zimonjic of Serbia in their first-round encounter, finally coming through 6-2 6-7 (6-8) 6-2.