NEW YORK >> At the end, and only at the very end, did Serena Williams face anything resembling a challenge in her U.S. Open semifinal.

Six times, Williams was a single point from winning. Six times, she failed to come through.

All that did, of course, was delay the inevitable Friday. On match point No. 7, Williams delivered a 107 mph service winner, then let out two shouts, a mixture of relief and rejoicing after a 6-0, 6-3 victory over fifth-seeded Li Na of China that put the defending champion back in the final at Flushing Meadows.

“I got a little nervous,” said the No. 1-ranked Williams, who will face No. 2 Victoria Azarenka of Belarus on Sunday in a rematch of last year’s final, “but I was able to close it out, finally.”

She usually does.

Pursuing a fifth U.S. Open championship, and 17th Grand Slam title overall, Williams has been so dominant, so untouchable, during these two weeks that the only question each time out was how long it would take her to win, not whether she would.

Through 12 sets across six matches in this tournament, Williams has lost a total of only 16 games (for context, Azarenka lost 13 in one match alone). The 31-year-old American can become the first woman to win the U.S. Open without dropping a set since — yes, you guessed it — Williams herself in 2008. She also did it in 2002.

Williams won 24 games in a row during a particularly perfect stretch that began in the second set of her fourth-round victory over No. 15-seeded Sloane Stephens, continued through a 6-0, 6-0 quarterfinal win against No. 18 Carla Suarez Navarro, and concluded with a 1-0 lead in the second set against 2011 French Open champion Li.

So what’s the secret to making things competitive against Williams?

“You’ve got to fight. You’ve got to run. You’ve got to grind. And you’ve got to bite with your teeth for whatever opportunity you have,” Azarenka said. “She’s obviously an amazing player. She’s the greatest of all time.”

Williams is definitely bolstering her case lately.

She is 66-4 with eight titles in 2013. Go back to the start of Wimbledon in June 2012, and she is 97-5 with 13 trophies, including three from the past five Grand Slam tournaments.

Half of Williams’ losses this season were to Azarenka, including one at a hard-court tuneup in Cincinnati last month.

“We know each other pretty well. I know her strengths; she knows my strengths,” Azarenka said. “That’s what it’s all about, about those turning points, who wants it more, who’s willing to go for it more.”

Azarenka has lost 12 of their 15 career meetings, but she did manage to push Williams to three sets a year ago in New York. This will be the first time the same women play each other in the U.S. Open final two years in a row since Williams lost to older sister Venus in 2001, then beat her in 2002.

Azarenka, a two-time Australian Open champion, improved to a tour-leading 31-1 on hard courts this season by overcoming all sorts of sloppiness to beat 83rd-ranked Flavia Pennetta of Italy 6-4, 6-2 in Friday’s first semifinal.

“I scared her a little,” said Pennetta, who kept a blue tissue box on one of her sideline chairs and occasionally went over to blow her nose between points.

Azarenka hit six double-faults, had far more unforced errors (25) than winners (15) and was broken in five of her nine service games. She’s one of the game’s top returners, however, and wound up with eight breaks of Pennetta, who was in her first Grand Slam semifinal at age 31.

Now comes a far tougher test against Williams, who had to go right back on court Friday night to team with Venus in the women’s doubles semifinals, where they lost 6-4, 6-2 to the Czech pairing of Andrea Hlavackova and Lucie Hradecka .

Earlier in the evening, with shadows stretching across the blue court and the Arthur Ashe Stadium lights on, it took Li 38 minutes of play to earn a game against Williams. And then — gasp! — she won another, breaking for a 2-1 lead in the second set with a big cross-court forehand to a corner that forced an error by Williams.

It was a jarring shift. It also was fleeting.

Good as Li is, with power off both wings and superb court coverage that helped her reach two Australian Open finals, she simply couldn’t stay with Williams. After Li went up 40-love to get within one point of a 3-1 advantage in the second set, she ceded six points with missed backhands to let Williams break right back. And Williams broke again to go up 4-2.

Li entered the day with a tournament-best 29 aces, but Williams did not allow one serve to get past her. Williams also tracked down ball after ball, using the nimble footwork that coach Patrick Mouratoglou says was the first thing he wanted to improve when they began working together 15 months ago.

Repeatedly, Li would appear to hit a point-ending shot. And repeatedly, Williams would somehow get it back over the net. That’s why Li finished with a paltry eight winners, 11 fewer than Williams.

“I need more practice,” Li said.

Williams leaves plenty of opponents feeling that way.

Still, there was the matter of winning Friday’s last point, and that took a while. With Li serving while down 5-2, they played an epic, 22-point game. Li erased match point No. 1 with a 101 mph service winner. On No. 2, Williams pushed a backhand wide. On No. 3, Li’s backhand down the line drew a forehand into the net. On No. 4, a 103 mph serve produced a netted forehand return. On No. 5, Williams ended a 10-stroke exchange by sailing a swinging forehand volley wide, then dropped to her knees. No. 6 ended with Li’s overhead winner.

When Williams sent a backhand long to lose the game after the nearly 1 1/2 -hour match’s most compelling 13 minutes, 49 seconds, some spectators responded with a standing ovation. If they were figuring there might only be one more game, they were right.

Serving at 5-3, 30-all, Williams watched Li’s backhand drop long, then bent forward and screamed, pumping her left fist. Once again, she was a point from the final, and this time she was able to seal the deal, thanks to a strong serve that Li could only put in the net.

Because of the way it finished, the match felt much closer than it ever really was.

During an on-court interview, Williams was asked about taking on Azarenka again.

“Well, we always have really good matches. I look forward to it,” Williams told the crowd. “It’s great to get to another final. She’s a great player and she lifts her game when it really counts.”

Sounds like a good description of Williams.