Kremlin: Champion Mikhail Youzhny was forced to pull out of the Kremlin Cup with a virus infection.
"I had made a decision to withdraw after losing my doubles match," the Russian world number nine told reporters.
"I don`t feel well at all and I don`t think I would have been able to play the singles in such condition."
Second seed Youzhny had a bye in the first round and was scheduled to play in round two on Thursday.
"I will now have to take a break from training for a few days and see how I feel," said Youzhny, still with a chance to qualify for the World Tour Finals in London next month.
"I`m scheduled to play next week in St Petersburg but right now I`m not sure if I`ll be able to be there. I`ll have to make a decision by the end of the week."
Youzhny`s pullout has added to the disappointments of the home crowd and local organisers. The annual $2 million dual ATP and WTA tournament has been decimated by the withdrawals of high-profile players, particularly on the women`s side.
Russians Vera Zvonareva, Svetlana Kuznetsova and Maria Sharapova and 2009 champion Francesca Schiavone all pulled out.
Serbia`s eighth seed Janko Tipsarevic, who lost to Youzhny in last year`s final, was bundled out in the first round on Wednesday by Horacio Zeballos 4-6, 6-4, 6-3.
Top seed Nikolay Davydenko takes on Uruguay`s Pablo Cuevas later on Wednesday.
On the women`s side, second seed Victoria Azarenka of Belarus beat Germany`s Andrea Petkovic 6-4, 2-6, 6-1 and eighth-seeded Spaniard Maria Jose Martinez Sanchez saw off Ukraine`s Alona Bondarenko 6-4, 1-6, 6-1 to reach the last eight.