ST. PETERSBURG, Russia — 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 champion was coming off a win Sunday at the Kremlin Cup, his fifth career title.

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.