Top-seeded Mikhail Youzhny of Russia retired with lower back pain while trailing Denis Istomin of Uzbekistan 6-2, 2-1 Thursday in the second round of the St. Petersburg Open.
The 2004 St. Petersburg champion, who won the Kremlin Cup for his fifth career title on Sunday, said the pain prevented him from playing his best in the first set.
Trailing 5-2, 30-15 on his serve in the first set, Youzhny double-faulted. He saved one set point but then returned wide to loose the set. He took a medical time out to have his back massaged but trailing 2-1, 15-40 in the second set he decided to retire.
Youzhny said he would decide if he would play at the Valencia Open next week after a medical exam.
In his second quarterfinal this season, Istomin, who has yet to win a title, will either face fourth-seeded Igor Andreev or two-time champion Marat Safin.
Earlier, Ukrainian qualifier Sergiy Stakhovsky beat Russian wild card Andrey Kuznetsov 6-1, 6-4, and Igor Kunitsyn defeated wild card Michail Elgin 6-2, 6-3 in an all-Russian match.
Stakhovsky broke Kuznetsov, the Wimbledon junior champion, three times to reach the quarterfinals of a tournament for the second straight week. He lost to Youzhny at the Kremlin Cup last week.
Stakhovsky will play German veteran Bjorn Phau, while Kunitsyn will face second-seeded Victor Hansecu of Romania.
