New York ? After trailing for much of the third set, Venus Williams suddenly was right back in the thick of her U.S. Open semifinal against Kim Clijsters, serving at 4-all, 30-all.

At that moment Friday night, it didn’t appear to matter that the 30-year-old Williams was bidding to become the oldest woman to win a Grand Slam title in two decades. Or that she arrived at Flushing Meadows coming off a left knee injury that meant she hadn’t played a match in more than two months.

Then came two pivotal points. First, Williams double-faulted for the seventh time, giving Clijsters a break point. Next, Clijsters curled a perfect backhand lob over the 6-foot-1 Williams to go ahead 5-4.

Williams stopped chasing and watched the ball fall, then hung her head. And that, basically, was that. Defending champion Clijsters held on, winning 4-6, 7-6 (2), 6-4 to extend her U.S. Open winning streak to 20 matches and return to the final.

“I just wish,” Williams said, “I could have played the bigger points a little better.”

She is 52-2 after taking the first set at the U.S. Open — and both of those losses came against Clijsters, who will face No. 7-seeded Vera Zvonareva of Russia in tonight’s final.

If Clijsters wins the championship, she will be the first woman with two consecutive U.S. Open titles since Williams in 2000-01.