Serena Williams kneeled on the court and scowled after a wayward forehand landed wide.
She chased down the ball with the ferocity of a player down a break in the third set. No matter that Williams had in fact won 11 of the match's 13 games — this relentless focus is what has returned her to the U.S. Open final.
Azarenka was far less sharp in beating 83rd-ranked Flavia Pennetta 6-4, 6-2 in the first semifinal.
"I know her game as well as she knows mine," Williams said. "She knows what I do great, what I do bad, and what I can do better. I know the same thing. At this point, it's just all about just playing some tennis now."
With Williams leading 5-2 in the second set, she and Li played a nearly 14-minute game spanning 22 points. Li fought off six match points, for one entertaining stretch providing the competitiveness that was otherwise mostly lacking.
"In the end, finally I can play tennis," Li said with a smile.
Li won the game but didn't extend her stay for long. The defending champion promptly served out the match.
Williams won 24 straight games from the second set of her fourth-round victory over 15th-seeded Sloane Stephens through the second set against Li, blanking No. 18-seeded Carla Suarez Navarro in between.
Li, the 2011 French Open champion, briefly made things interesting, breaking in the set's third game — just the second time all tournament Williams had been broken.
Then Williams broke Li in her next two service games to restore order.
"I think I started out really well and I was really focused," Williams said. "I think Li Na may have been a little off, but I think it was because I started out so well and then she may have not played her best. But I think she played well. It was, especially toward the end, a very quality match."
The 31-year-old American is one win from a fifth U.S. Open championship and 17th Grand Slam title overall. But if there's one player not intimidated by all those stats, it's Azarenka, who pushed her to the brink in last year's final.
Then again, the Belarusian has hardly looked up to it at Flushing Meadows so far in 2013.
Half of Williams' losses this season are to Azarenka, including one at a hard-court tuneup in Cincinnati last month.
"The battles that we had, it was really just taking each other out of the most comfortable zone and just fight for every ball," Azarenka said.
Williams will be coming off a loss when they meet Sunday. Just over an hour after she beat Li, she returned to the court for a doubles semifinal with sister Venus.
They fell 6-4, 6-2 to the Czech team of Andrea Hlavackova and Lucie Hradecka.
Azarenka has looked shaky at times at the U.S. Open against list of foes not exactly of Williams' caliber: This will be her first opponent ranked in the top 14.
Azarenka, the two-time Australian Open champion, was broken five times in nine service games by Pennetta, who was in her first Grand Slam semifinal at age 31.
Williams has dropped 16 games in six matches; Azarenka lost 13 in her fourth-round win over Ana Ivanovic alone.
"I definitely feel like when she plays me she plays her best, by far," Williams said. "I have seen her play other players, and when I play her I'm playing a totally different player."
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