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 tight, which happens sometimes. I just needed to relax, and then when I did relax, she played some good points," 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 it was a good experience going into the next round," Williams added. "If I even get a match point, I'll be ready to stay calm and just to stay focused and relaxed."

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 only 16 games (for context, Azarenka lost 13 in one match alone). According to the WTA, that's the second-lowest total of dropped games through six matches at any Grand Slam tournament in the last 25 years.

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.

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.

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.

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