NEW YORK — So little to separate them over their careers, so little to separate them on this night.

Serena Williams barely got the better of older sister Venus Williams in a U.S. Open quarterfinal that was fit for a final, coming back in each set to win 7-6 (6), 7-6 (7) last night and break a tie in their head-to-head series.

Serena trailed 5-3 in both sets. She faced set points in both — a total of 10, including eight in the second. But she advanced to the semifinals at Flushing Meadows for the first time since 2002, the year she beat Venus in the title match for her second U.S. Open championship.

"I felt like I was always in control," Venus said. "If it was someone else, I definitely feel like I would have won the match."

It was the siblings' 17th meeting as professionals, and Serena leads 9-8. That includes 11 matches at Grand Slam tournaments, where Serena leads 6-5.

She also has the edge in major championships, 8-7, and only she can add to that total this weekend.

"It's really just unfortunate it had to be in the quarters," Serena said.

In tomorrow's semifinals, the fourth-seeded Serena will meet No. 6 Dinara Safina, who advanced earlier in the day by overpowering No. 16 Flavia Pennetta, 6-2, 6-3. No. 2 Jelena Jankovic will face No. 5 Elena Dementieva in the other semifinal.

The seventh-seeded Venus had all sorts of chances to take control, but in the end, as both women's play reached a very high level, it was Serena who pulled through. In the second tiebreaker, Venus had four set points — and Serena saved them all.

"I'm a very good closer," Venus said, "so today was, um — I've never had a match like this in my life, so I guess there's always a first."

The start of the latest all-Williams showdown was delayed by more than an hour because of two lengthy matches that preceded it on the tournament's main court, including a women's doubles match and No. 6 Andy Murray's four-set victory over No. 17 Juan Martin del Potro in the men's quarterfinals.

The long day was capped by No. 1 Rafael Nadal beating unseeded American Mardy Fish, 3-6, 6-1, 6-4, 6-2, getting past the quarterfinals for the first time in six trips to Flushing Meadows.

The match started at 11:30 p.m. and ended at 2:10 a.m. local time, making it the third-latest finish in tournament history.

Venus showed up at the locker room about 20 minutes before she and her sister finally headed out, carrying a bunch of rackets in the crook of her left arm. Serena arrived about five minutes later, a red purse slung over her left shoulder.

Neither face betrayed the slightest hint of emotion, and those same expressionless masks were in place at the match's start. Early on, there were the sorts of nerves and erratic play — a combined seven first-set double-faults, for example — that have marked many of the siblings' encounters as they have adjusted to playing one another.

"I try not to look at her, because if I look at her, I might start feeling sorry," Serena told the crowd afterward.

Neither of their parents, who also serve as their coaches, were sitting in the guest boxes at Arthur Ashe Stadium. An older sister, Isha Price, was there, sitting with her hands clasped in front of her face, eyes shut, during the first-set tiebreaker.

How could she possibly cheer for one sister against another?

"I was stressed," she said. "It is so difficult to watch them. At the end of the day, you want them to play a good match and for the best person to win."

And did the best person win?

"I'm not sure," Price said.