NEW YORK — At the end, and only at the very end, did Serena Williams face anything resembling a challenge in her US 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. On match point No. 7, Williams delivered a 107-miles-per-hour 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 US 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 US 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’s losses this season were to Azarenka, including one at a hard-court tune-up 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 US 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 83d-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 Friday night to team with Venus in the women’s doubles semifinals, although the sisters lost to Andrea Hlavackova and Lucie Hradecka, 6-4, 6-2.