Things are coming together rather perfectly at the U.S. Open for Simona Halep, the pint-size Romanian with a gallon of game. It doesn't hurt, either, to have the Perfect 10 in your player's box, gymnast Nadia Comaneci.

Comaneci, the Romanian who became the first to earn a perfect score of 10 in gymnastics at the 1976 Olympics, was cheering on Halep on Wednesday at Arthur Ashe Stadium as she won her quarterfinal match over Victoria Azarenka, 6-3, 4-6, 6-4. Halep advanced to 's semifinals, where she will play Flavia Pennetta, a 4-6, 6-4, 6-2 winner over Petra Kvitova.

Halep is the Open's No. 2 seed and has been probing the heights of Grand Slam tennis for the past two seasons. She finished runner-up last year in the French Open. She has won three titles this season, including the big one at Indian Wells.

The 23-year-old, generously listed at 5-6 and 132 pounds, brings an array of weapons to the court, including a surprisingly effective serve. But it's her precise, deep ground strokes, set up by her scurrying speed, that do the most damage, and she is one of the best at painting the lines.

On Wednesday she said she benefited from some advice from her coach, Darren Cahill, during the Open's first rain delay of about an hour and a half that came with Halep serving in the third set. She said she had a bunch of negativity in her head after the second-set loss, but came out energized after the break and immediately served an ace to hold.

"The rain delay was perfect for me," Halep said. "It was unbelievable I had another chance."

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What was also perfect was having Comaneci on her side.

"She was a great champion. She had a perfect 10, first 10," Halep said. "To have a great champion in your box, it gives you power, that she appreciates what I am doing . . . She was great and I really want to thank her for coming. She told me that she's coming back tomorrow again."

Pennetta outlasted Kvitova in sticky, hot conditions in the earlier quarterfinal. In the first set, Pennetta broke Kvitova to recover an earlier break of her service. She went up 40-love in the 10th game and lost the next five points, double-faulting on the final one to hand Kvitova the set.

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Kvitova struggled with the conditions all day, magnified by a case of mononucleosis that she was diagnosed with in August. And although she was ahead in the second set, it seemed all she could do was just to play it out. Pennetta broke her in the ninth game, and served out the set, though she had to fend off a break point.

Pennetta remained the steadier of the two players in the third set as Kvitova clearly wore down and had little left at the end as Pennetta closed it out. As she had all match, Kvitova tried for winners to shorten the rallies, but in the end made a ton of errors. Kvitova made 50 groundstroke errors compared with 12 for Pennetta.

"From the beginning of the match I didn't really feel 100 percent today," Kvitova said. "I'm just glad that I left everything [on the court]. I suffered a lot, but I'm still here. That's good."

Like her compatriot, Roberta Vinci, Pennetta never expected to get this far at the Open. But here they are, Pennetta 33 and Vinci 32, in the semis.

"Before the tournament I never think to be so far in the tournament, so it's special," Pennetta said. "It's something amazing for me in this moment."