TORONTO (AP) -- Svetlana Kuznetsova, who begins her U.S. Open title defense in 10 days, played despite lower back pain Thursday and lost to Argentina's Gisela Dulko 7-6 (3), 7-6 (8) at the Rogers Cup.
The third-seeded Russian struggled throughout the match, and in the first set it appeared she would be unable to finish.
"It's hurting very much. I think I should have stopped playing,'' Kuznetsova said. "But when the match is so close, it's so difficult just to say go, and that's it.''
Dulko, ranked 35th, forced Kuznetsova several times to lunge for crosscourt shots. Trailing 6-5 in the first set, Kuznetsova asked a trainer to massage her back. She walked gingerly back onto the court, clutching her back and grimacing.
"I said, 'OK, I'll play two more games and see what's going on,''' Kuznetsova said. "I couldn't play my backhand at all. My forehand, I can move only the hand. I cannot turn my body. I could run more or less.
"But I just couldn't take this decision to pull out. I never did it in my life.''
Between sets, the Russian headed for the locker room, returning in a long-sleeve shirt. Her back seemed to loosen midway through the second set, but Dulko persisted and closed the match with a blistering forehand to reach the quarterfinals.
"My game plan was to move her a little bit,'' said Dulko. "I think I did it pretty well. We both ran a lot. I think it was a tough match for both of us.
"She fought until the last point. It was a really tight match. And I think yes, it's a really big win for me because I get the opportunity to be in the quarterfinals of this big tournament.''
Kuznetsova became the first Russian woman to win the U.S. Open singles title last year, beating Elena Dementieva in the all-Russian final.
Injuries have become the theme at the Rogers Cup, and tournament directors received more bad news Thursday when 12th-seeded Ana Ivanovic withdrew because of a strained right chest muscle.
"It hurt during the doubles match (Wednesday) night,'' Ivanovic said in a statement. "The best thing I can do now is to get treatment and hopefully be ready for the U.S. Open.''
Ivanovic was scheduled to play Kim Clijsters of Belgium on Thursday afternoon. Clijsters, the No. 7 seed, walked into the quarterfinals, where she'll play 14th-seeded Flavia Pennetta. Pennetta advanced when Serena Williams pulled out Wednesday.
Ivanovic became the seventh top-20 seeded player to withdraw. Williams, the No. 5 seed, pulled out with a knee injury, top-seeded Russian star Maria Sharapova backed out Sunday due to a strained chest muscle, and Williams' sister Venus, ranked ninth in the world, withdrew last week due to the flu.
Mary Pierce of France, the No. 8 seed, pulled out Tuesday with a strained right quadriceps muscle.
There were more tense moments when Anastasia Myskina rolled her left ankle in the second set of her match against Shinobu Asagoe.
But Myskina, the No. 9 seed, had the ankle taped and went on to beat Asagoe 7-6 (5), 7-5. Myskina will play Dulko in the quarterfinals.
In other matches, No. 2 seed Amelie Mauresmo cruised by Conchita Martinez 6-0, 6-2 to advance to the quarters, and No. 4 seed Justine Henin-Hardenne eliminated China's Jie Zheng, 6-2, 6-4. Henin-Hardenne will play Nicole Vaidisova in the quarters, after Vaidisova beat countrywoman Hana Sromova 6-2, 6-2.
"Obviously, just not spending too much time on the court is an advantage when you play these kind of tournaments, when you have to go out every day,'' Mauresmo said.
She will meet Nadia Petrova, who moved on after Li Na had to quit her match in the second set due to an ankle injury. Petrova was leading 6-3, 3-0.