Second seed Svetlana Kuznetsova [ Images ] 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 [ Images ] 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 [ Images ], who beat Kateryna Bondarenko of Ukraine 6-2, 6-3.

In the other half of the draw, third seed Jelena Jankovic [ Images ] 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 [ Images ]. 

Dementieva's opponent, fellow Russian and fifth seed Nadia Petrova [ Images ], 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 [ Images ], reigning champion Martina Hingis [ Images ] and fourth seed Nicole Vaidisova [ Images ] -- 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 [ Images ], which starts on May 27.

Eighth seed and 2002 Rome champion Serena Williams [ Images ] 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 [ Images ] 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 [ Images ] Patty Schnyder [ Images ], who showed the same composure that took her to a runner-up finish in Rome in 2005 by beating Australian Samantha Stosur [ Images ] 6-4, 6-4.