NEW YORK — 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) tonight and break a tie in their head-to-head series.

Serena trailed 5-3 in both sets. She faced set points in both, 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.

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. The fourth-seeded Serena will meet No. 6 Dinara Safina in the semifinals.

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

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.

Then, when Serena earned her first match point, nearly 2½ hours into the match, she converted it, when Venus ended an 11-stroke exchange by missing a forehand.

Back when they were ranked Nos. 1 and 2, the siblings only could meet in tournament finals. But because of injuries, inactivity and inconsistency, they dropped in the rankings, and now it's the luck of the draw that determines at which stage they potentially meet.

At Wimbledon in July, for example, the wound up on opposite halves of the field, and Venus beat Serena in the final for her fifth title at the All England Club. At the U.S. Open, they wound up in the same section of the bracket, so the women many consider the two top players at the moment were forced to meet in the round of eight.

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.

Venus showed up at the locker room about 20 minutes before they 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. "I want the best for her. I love her so much. She's my best friend."

Neither of their parents, who also serve as their coaches, were sitting in the guest boxes at Arthur Ashe Stadium. One of their sisters 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?