Second seed Svetlana Kuznetsova had to work harder than expected to reach the quarter-finals of the Italian Open on Thursday. She beat Yuliana Fedak of the Ukraine 7-6, 6-1.

Kuznetsova, the highest-ranking player left in the claycourt tournament after the elimination of top seed Amelie Mauresmo on Wednesday, twice failed to serve out for the first set and was 4-1 down in the tiebreak before finding her rhythm.

Fedak, ranked 138 in the world, had entered the main draw only as a lucky loser from qualifying.

"It was very hard for me. This is my third week in a row playing tennis and I felt so uncomfortable on the ball in the first set, so it was very frustrating," Kuznetsova said at the courtside.

"But I knew I could play better, so I just tried to concentrate and in the end I started to play better."

Kuznetsova's reward was a last-eight meeting against last year's Rome runner-up, Russian Dinara Safina, who beat Kateryna Bondarenko of Ukraine 6-2, 6-3.

In the other half of the draw, third seed Jelena Jankovic advanced by edging out Bondarenko's older sister Alona 6-4 7-5 to set up a last eight meeting with 10th seed Elena Dementieva. 

Dementieva's opponent, fellow Russian and fifth seed Nadia Petrova, retired with a back problem at 4-3 and 15-0 down in the first set.

Petrova became the latest top player -- after world number two Maria Sharapova, reigning champion Martina Hingis and fourth seed Nicole Vaidisova -- to pull out of the tournament with an injury.

"I had an accident in the gym. I fell on the floor and it was very sore. Today I ran for a wide ball and I felt a pain right there," Petrova said after receiving treatment at the courtside.

"It's a shame. I was really hoping to go far this week. Even today I was showing good tennis."

Petrova did not say whether the injury would prevent her taking part in the French Open, which starts on May 27.

Eighth seed and 2002 Rome champion Serena Williams also reached the quarter-finals, overcoming a halting start to find the range and rhythm on her groundstrokes and ease past Shahar Peer of Israel 6-3, 6-3.

Williams had been pushed to 8-6 in a deciding set by Peer on her way to winning the Australian Open in January.

"This was definitely a good test. We had such hard matches in the past and I knew she would be a tough opponent, especially on clay where she gets a lot of balls back," the former world number one said.

Williams next plays Switzerland's Patty Schnyder, who showed the same composure that took her to a runner-up finish in Rome in 2005 by beating Australian Samantha Stosur 6-4, 6-4.

• Complete tennis coverage
• All you want to know about Sania Mirza