Top seed Janko Tipsarevic came from round match Thursday at the $1 million Kremlin Cup tennis event.
The 2009 Moscow runner-up overcame Russian Igor Kunitsyn 6-7 (5-7), 6-1, 6-0 in 1 hour, 55 minutes on the indoor hardcourts at Olympic Stadium.
"In the first set I wasn't hitting the ball enough. I was mentally there, I was fighting, but I wasn't aggressive," said the Serbian Tipsarevic.
Kunitsyn was the surprise champion here three years ago.
Tipsarevic, who captured his first-ever ATP title in Malaysia earlier this month, will face Russia's Dmitry Tursunov in Friday's quarterfinals. The sixth-seeded Tursunov got past Frenchman Julien Benneteau 6-3, 7-6 (7-1) on Day 4.
Upsets came when France's Jeremy Chardy took out third-seeded Ukrainian Alexandr Dolgopolov 6-3, 7-6 (8-6) and German Philipp Kohlschreiber doused seventh-seeded Gilles Muller of Luxembourg 7-6 (7-4), 6-3.
Friday's other quarters will pit second-seeded and defending Moscow champion Viktor Troicki against fifth-seeded Russian-born American Alex Bogomolov Jr. fourth-seeded three-time Moscow champ Nikolay Davydenko versus German qualifier Michael Berrer and Kohlschreiber against the qualifier Chardy.
The Serbian Troicki topped Cypriot Marcos Baghdatis in last year's Moscow finale. The former top-five star Davydenko, of Russia, titled here in 2004, 2006 and 2007, as Russians have dominated this event, capturing 14 of the 21 titles here.
The 2011 Kremlin Cup titlist will claim $132,160.