By Matthew Cronin

SAN DIEGO, Aug 3 (Reuters) Defending champion Mary Pierce was made to struggle in her first competitive match for six months before progressing to the third round of the Acura Classic in San Diego.

The 31-year-old sixth seed, who has not played since February after tearing ligaments in her foot, edged past Austria's Sybille Bammer 6-1 6-7 7-6 yesterday.

Third seed Nadia Petrova was not so fortunate in her first match in over two months when she was upset 6-3 6-7 6-2 by fellow Russian Anna Chakvetadze.

Top seed Kim Clijsters crushed Australia's Samantha Stosur 6-1 6-2 in the second round.

Pierce struggled with her form much of the day, throwing away three match points with forehand errors in the second set tiebreaker, which she lost 9-7.

But the two-times Grand Slam champion stood strong in the third set against the left-handed Austrian, mixing heavy blows from the baseline with occasional net rushes.

''I got a lot of tennis in,'' said Pierce. ''Three sets, two tiebreakers, being ahead, being down. It's a good start. The most important thing is I know I can play without hurting myself. I feel like there's not much missing from my game.'' Pierce reached two Grand Slam finals last year at the French and US Opens and, despite her injury, said she still has some major goals left.

''The only thing I haven't done is been ranked number one, made the final of Wimbledon or win it, and I don't have an Olympic medal,'' Pierce said. ''Those are the three things I want.'' Petrova injured her hip prior to Roland Garros and has not played since her first-round exit there to Japan's Akiko Morigami and looked rusty against the 19-year-old Chakvetadze.

''I felt absolutely out of it,'' said Petrova. ''The ball was just flying by me. I also had a tough time deciding which shot I should choose.

''I didn't know whether to go crosscourt, down the line, where I should serve. She played okay, but if I played at my level, it would have been much different.'' Chakvetadze will play another teenager, Serbia's Ana Ivanovic, who destroyed American qualifier Amy Frazier 6-3 6-2.

Fourth seed Elena Dementieva took down American Ashley Harkleroad 6-2 6-2 and will face India's Sania Mirza, who upended 15th seed Katarina Srebotnik 6-3 7-6.

Russia's Vera Zvonareva advanced when China's Peng Shuai retired with heat illness. Peng had won the first set 6-3 and was two points from winning the match in the tiebreaker before she lost the second set 7-6 (7-5) and then retired.

Zvonareva will now face second seed Maria Sharapova.

Flavia Pennetta of Italy, seeded 12, overcame American Jamea Jackson 6-2 7-6 and will play eighth seed Martina Hingis.

It is the first time since the tournament began in 1984 there are no American women in the third round.

Lindsay Davenport and the Williams sisters withdrew before the event with injuries.

REUTERS PM BS1208