Serena toils in Rome

The world number one admitted that she was still blowing away the cobwebs from her long absence recovering from a knee injury after toiling to beat Germany's Andrea Petkovic 6-2 3-6 6-0.

Russian Kirilenko also needed three sets to book her place in the quarter-finals, recovering from a set down to beat Slovakia's Dominika Cibulkova 4-6 6-1 6-1.

The American had a sticky patch against unseeded Petkovic, after having taken her time to get going against Swiss Timea Bacsinszky on Tuesday in her first match since winning January's Australian Open.

However she has already shown in Rome an ability to raise her game when needed, an attribute she is likely to have to call upon at this month's French Open.

"I don't get frustrated because I have to think I've had a long time off," she said. "I definitely felt extremely rusty today.

"It went crazy in the second set. I was glad I was able to win because at least I can be guaranteed another match. I think I definitely need it because I was definitely rusty."

Serena looked comfortable in the first set, reading her opponent's beefy serve well and using Petkovic's power to crack some sizzling return winners, although only a smattering of Romans had braved the drops of light rain to see them.

She was swinging the racket with gusto at the start of the second set, with a gorgeous net volley and an emphatic smash helping her to hold to love in the second game.

Serena then began attacking the ball too early, leading to unforced errors that let the German off the hook when she was under pressure in the following game.

The world number one's rhythm suddenly went astray and she was broken twice on the way to losing the second set, bouncing her racket on the ground in frustration when a netted forehand clinched it for Petkovic.

Serena regained her concentration though to win the decider without dropping a game.

With an eye on Roland Garros, she said that, despite the problems, her claycourt game was better than at the same stage last season, when she lost here in her first game.

Denmark's world number two Caroline Wozniacki, who has been struggling with an ankle problem recently, was thoroughly outfoxed in the red-dirt arts by Spaniard Maria Jose Martinez Sanchez, crashing out 6-4 6-2.

Fourth seed Venus Williams, back from injury like her sister Serena, progressed into the last eight with a 6-3 6-4 win over Shahar Peer that was harder than the score suggests.

Venus, the 1999 champion, looked like she might have snapped the Israeli's resistance when she took serve in the fifth game of the second set.

But the 16th seed broke back immediately and Venus had another battle in her next service game before snatching the breakthrough in the one after with a forehand winner that clipped the edge of the line.

In the next round Venus will face twice former champion Jelena Jankovic of Serbia, after the seventh seed thrashed Belgium's Yanina Wickmayer 6-2 6-0.

Jankovic's compatriot Ana Ivanovic, who has slipped to 58th in the rankings after being top two years ago, continued her return to form with a 6-1 7-6(5) victory over Russian world number six Elena Dementieva.

Ivanovic came into the tournament without a win over a top 10 opponent in 18 months but she has now notched two in as many days after beating Victoria Azarenka on Tuesday.