NEW YORK — Forget about forehands and backhands. Melanie Oudin's biggest weapon is her heart.

The 17-year-old sparkplug from Georgia proved it again Monday at the U.S. Open, extending her remarkable run to the quarterfinals with another come-from-behind victory, 1-6, 7-6 (2), 6-3 over 13th-seeded Nadia Petrova.

Oudin staved off four points that would have put her behind 5-3 in the second set, then rolled through the third, hitting corners with those underrated groundstrokes and taking advantage of 22 unforced errors by her more-seasoned, higher-ranked opponent.

Rankings, like her age, however, are only numbers.

The 70th-ranked player already had wins over No. 4 Elena Dementieva and No. 29 Maria Sharapova at Flushing Meadows, along with one over former No. 1 Jelena Jankovic this summer at Wimbledon. Now, she's knocked off No. 13.

After hitting a forehand to the corner for her 19th and final winner on match point, Oudin threw her racket in the air. This time, however, she didn't cry any tears of disbelief. Instead, it was twin sister Katherine sobbing in the stands.

"I'm so happy to be in my first quarterfinal Grand Slam everrrr," Oudin told the crowd in her postmatch, on-court interview.

Talk about heart. Oudin improved to 6-1 at Wimbledon and the U.S. Open this year when she's lost the first set. She is 17-4 overall this year in three-set matches.

"I started serving better, thought I could do it and — I did," she said.

But Oudin won this match without serving a single ace, another indication she is not blowing anybody away with sheer power. Instead, it's footwork, technique, precision. Mostly, though, she is thriving in pressure situations that make so many others in the sport shrink away.

She stung two forehand winners, including one when she was positioned completely outside the court, to take a quick 3-0 lead in the second-set tiebreaker. She also took advantage of three unforced errors and a double-fault from her 27-year-old opponent, who was trying to make her third Open quarterfinal.

Too young to know any better? Maybe. Regardless, she has become the youngest American to move into the quarterfinals at America's Grand Slam since Serena Williams in 1999.

"I think this is going to do a lot," Oudin said. "I think it's good for American tennis."