Maria Sharapova emerged from a bathroom break last night with a look of annoyance and determination on her face. Her father, Yuri Sharapov, had been screaming at her from his box, shouting out instructions and gesticulating wildly with his hands. His daughter’s first U.S. Open semifinal was at stake, and it was time for her to clean up her act.

Sharapova responded just in time, though perhaps not as forcefully as she would have liked. In the end, her victory was more of an escape, as the 18-year-old Russian and top seed wriggled and screamed her way through the third set of an uneven match against Nadia Petrova, prevailing 7-5, 4-6, 6-4.

Petrova, not known for stellar performances under pressure, set her own downfall in motion. She stayed in the match with a service break, but she could not capitalize on a 40-15 lead to level the third set at 5-5. First, she double faulted. Then she dumped a backhand into the net. Sharapova produced a smart rally at deuce, working Petrova into the backhand corner and coming away with another error. On the final point of the match, Sharapova let loose a little magic, lunging for a backhand service return and blocking it crosscourt. Petrova dumped another backhand into the net, and Sharapova let out a shriek.

“It’s absolutely amazing, I can’t believe I pulled this match out today,” Sharapova said.

Over the first two sets of tennis inside Arthur Ashe Stadium, Sharapova blasted 22 unforced errors, greeting many of them with a shrug, a self-deprecating remark, an abbreviated leap of surprise, or a dismissive swipe of her arm. Despite stealing the first four games from Petrova, after nearly two hours she found herself even at a set apiece. She even engaged in an argument with the chair umpire after a tennis ball bounced into the court, seemingly from the stands, as Petrova lined up a winner. Despite the disturbance, play was allowed to continue.

In the third set, Sharapova strode back to the court from her brief break in the locker room and blasted her way to a 2-0 lead. Each player held serve until the last two games, though there were precarious moments, from flubbed volleys to wild strokes. In all, Sharapova hit 48 unforced errors, while Petrova smacked 50.

For Petrova, the loss was another heart-wrenching chapter in a six-year career known more for its disappointments than its successes. The superbly athletic Russian scored a satisfying victory in Flushing last year against then defending champion Justine Henin-Hardenne, but she has never been able to keep her imposing serve and powerful strokes together long enough for a title in any tournament.

For much of last night, Petrova seemed ready to discover herself. Though neither player could be accused of committing pretty tennis, Petrova was the front-runner in terms of grit. Just making the first set a contest was a feat in itself. In the end, it only made her collapse all the more discouraging.