NEW YORK -- Serena Williams was cruising along in the U.S. Open semifinals, serving up another bagel, when, out of nowhere, a competitive tennis match broke out.
Didn't last long.
The defending champion won the first seven games of Friday's match, then actually fell down a break early in the second set, before quickly regaining it on the way to a 6-0, 6-3 victory over fifth-seeded Li Na.
The day's only drama came in the second-to-last game -- an affair that went eight deuces and lasted 13 minutes, 49 seconds. Li saved six match points, but it only delayed the inevitable. Williams dropped two points in the next game, but ended it and let out a long, celebratory scream after a 107 mph service winner.
"It got tough at the end. I got a little nervous, but I was able to close it out finally," Williams said.
The victory set up a rematch of last year's final against Victoria Azarenka, who beat 83rd-ranked Flavia Pennetta 6-4, 6-2 in the earlier semifinal.
The first set of the Williams match marked the third straight set the No. 1 seed won at love, and when she held serve to open the second, she had won her 24th straight game of the tournament, dating to her match last Sunday against Sloane Stephens.
Nothing lasts forever, of course, though Williams is extending her string of dominance in the women's game deep into a second decade.
Sunday's match will mark her 21st Grand Slam final. She stands one win away from capturing her fifth U.S. Open title and 17th major trophy. This will be the first time the same two women have met in back-to-back finals at Flushing Meadows since it was Williams against her sister, Venus, in 2001-02. It will also mark the first 1 vs. 2 final at the U.S. Open since 2003, when No. 2 Justine Henin beat No. 1 Kim Clijsters.
Azarenka lost to Williams in a tense three-setter last year. But just last month, she beat Williams in a third-set tiebreaker in the final of a tune-up tournament in Cincinnati.
"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. It's a bunch of combinations."
Williams took a brief break then turned around to play in the semifinals of the doubles with her sister. She is trying to make the finals of the singles and the doubles at the same Grand Slam for the first time since last year at Wimbledon.
For her match against Li, Williams wore pink shoes embroidered with tiny hearts on the heels, each of them with a number -- `99, `02, `08, `12 -- marking each year she hoisted the trophy in Queens.
She's showing every sign she'll need to add another heart , and Azarenka is the only one standing in the way.
"We always have really good matches. I look forward to it," Williams said. "It's great to get to another final. She's a great player and she lifts her game when it really counts."
Azarenka, the 2012 and 2013 Australian Open champion, is trying to become the first woman to win the majors in Melbourne and New York in the same year since Martina Hingis in 1997.
If Friday's proceedings on a cool, breezy day in Arthur Ashe Stadium were any indication, Azarenka has some work to do.
Hours before Williams overpowered Li, the 2011 French Open champion and one of the few players with the power to hang on the court with the world's No. 1, Azarenka won a push-fest against Pennetta. She won despite serving six double-faults as part of her 25 unforced errors, against only 15 winners.
There were only five service holds over 18 games, though Azarenka got four of them.
"It was a little bit tricky," Azarenka said. "I couldn't find my rhythm at the beginning. I felt I was rushing too much. I couldn't really put the ball in play. It's good I stayed tough, played well in the return game at important moments."
In the late match, Li only hit eight winners against Williams -- evidence of how the world's top player can keep anyone, even some of the biggest hitters, off balance.
Williams also served four aces and hit 19 winners of her own -- 15 of which came in the second set, after Li briefly made things competitive.