MOSCOW (AFP) - Defending champion Anastasia Pavlyuchenkova reached the Kremlin Cup quarter-finals on Wednesday while US Open winner Flavia Pennetta pulled out through injury despite also successfully booking her last-eight spot.

Pavlyuchenkova, 29th in the world, defeated Czech second seed Lucie Safarova 6-1, 6-7 (2/7), 6-3 to record her fifth win in seven meetings against the left-handed French Open runner-up.

"I'm really tired," said Pavlyuchenkova, who last week won her first title of 2015 at Linz.

"I felt exhausted ahead of today's match. I have no idea how I managed to win it."

Pavlyuchenkova took the opening set in 35 minutes and grabbed a commanding 5-2 lead in the second.

But Safarova, who needed to reach the semi-finals to claim a place in the season-ending WTA Finals, battled back to send the set into a tiebreak, which she won.

Early in the deciding set, Pavlyuchenkova took a medical time-out because of the problems with her ribs.

But the 24-year-old Russian still managed to regain the initiative to take the tie.

Italian third seed Pennetta cruised comfortably into the last eight with a 6-2, 6-4 win over Russia's Daria Gavrilova which guaranteed her place in the WTA Finals in Singapore.

But an hour later, the US Open champion withdrew from the rest of the Moscow event due to problems with her right foot.

Pennetta's withdrawal handed Ukraine's Lesya Tsurenko a walkover into the semi-finals.

In the ATP section of the event, second seeded Spaniard Roberto Bautista Agut, last year's runner-up, cruised into the last eight, disposing of Turkey's Marsel Ilhan 6-2, 7-5.

Third seed Serb Viktor Troicki, the 2010 champion, lost to Russia's Teymuraz Gabashvili 6-2, 6-3.

Dutchman Robin Haase ousted Uruguayan fifth seed Pablo Cuevas, 6-2, 5-7, 6-1.