About 20 minutes after Justine Henin won last night’s much anticipated quarterfinal match at the U.S. Open, Serena Williams seemed unable to come to grips with what was painfully obvious to everyone in Arthur Ashe Stadium: She is not the best tennis player in the world, not right now.

That honor, without doubt, belongs to Henin, the 5-foot-6 Belgian dynamo who traded punches with the more powerful Williams for a set last night and then knocked her out cold, 7–6(3), 6–1. Williams, who lost to Henin at Wimbledon but had a sprained thumb to dull the pain of defeat, offered no injuries or excuses last night, though she couldn’t let go of the idea that this match — and every match — was hers to win if she played her best.

Five years ago, that was certainly the case. It’s not now. Henin demands more of Williams than any woman, other than her sister Venus, ever has.

Williams, in a foul mood, wasn’t up for the discussion.

“I don’t want to get fined,” she told a group of reporters at a press conference that lasted less than 10 minutes. “That’s the only reason I came.”

Williams congratulated Henin on her play but couldn’t resist saying the Belgian had won because “she made a lot of lucky shots, and I made a lot of errors.”

Henin is the sort that makes her own luck. She’s smaller than most women on the tour, but she trains and practices like a champion and hits as hard, and moves as well, as anyone. She’s been bothered by a weak shoulder of late, and she hardly served her best last night. But she scurried about and often forced Williams into awkward positions at the net and along the baseline.

After a tense first set in which Henin blew a lead and faced down set point, she raced ahead in the second set and then stomped on Williams, winning the last three games with a flurry of backhand winners, overheads, and perfectly placed forehands. Williams, still in less than perfect condition, looked sullen as it became clear that the form that carried her to a title at the Australian Open this year would not be enough to stop Henin, even under the lights at her favorite tournament. Henin weathered one last Williams onslaught in the middle of the second set and then sprinted to victory.

Every match these two women play has meaning beyond the tournament at hand. Henin and Williams, both 25 years old, are the best players of their generation. Williams has eight major titles, Henin six. In the last two years, Henin has won two major titles and been to the finals of five. The last 18 months have been the best of Henin’s career, and 2007 has been Williams’s best year since the days she dominated tennis and won four consecutive majors, though not in the same season.

Henin now has the chance to do something that hasn’t been done at a major tournament since 2001: defeat both Williams sisters in the same event. If Venus Williams, who has looked incredibly strong so far, defeats Jelena Jankovic tonight, Henin will have her chance in the semifinals on Friday. Venus will pose a bigger challenge for Henin, as she volleys better than Serena and plays better defense. Venus has won seven of their eight meetings, but the two have not met since 2003, before Henin had won the first of her major titles. She’s a different woman now, and seemingly on her way to executing one of the best performances this tournament has ever seen.