That's because Azarenka, a two-time major champion and former No. 1 in her own right, was playing spectacular tennis, too, nearly the equal of Williams in every facet.
For when Williams finds her best game, she becomes unbeatable. And for her past 26 Grand Slam matches she is, indeed, unbeaten. Erasing an early deficit at Centre Court, Williams got past Azarenka 3-6, 6-2, 6-3 Tuesday with the help of 17 aces and a remarkable ratio of 46 winners to 12 unforced errors.
"It's been up and down, up and down, but somehow I'm still alive. I don't know how," said Williams, who twice was two points from losing to Britain's Heather Watson in the third round and is now 14-0 in three-setters and 37-1 overall in 2015. "So we'll see what happens, but I'm just happy to still be here."
She is closing in on a fourth consecutive major title for a self-styled Serena Slam, which she already accomplished in 2002-03. Pull that off, and Williams also will have the third leg of a calendar-year Grand Slam and go to the U.S. Open with a chance to become the first player since Steffi Graf in 1988 to win all four major trophies in one season.
"I haven't seen her play like this, honestly," said Azarenka, who has lost 17 of 20 matches against Williams and all 10 meetings at majors, including after leading by a set and a break at the French Open in May.
In Thursday's semifinals, No. 1 Williams faces No. 4 Maria Sharapova, who beat unseeded American CoCo Vandeweghe 6-3, 6-7 (3), 6-2.
Williams is 17-2 against Sharapova, including 16 straight victories. But one of Sharapova's wins came at Wimbledon, in the 2004 final, when at age 17 she stunned Williams for the first of her five Grand Slam titles.
"Definitely no secrets between each other's games," Sharapova said.
Williams, whose major trophy count is at 20, said of the matchup: "I look forward to it."
Here was the scouting report from Williams' coach, Patrick Mouratoglou: "If she plays like today, I don't think anyone can compete."
Pick an adjective for Williams-Azarenka. Intense. Riveting. Entertaining. Sublime. For 2 hours, 4 minutes on a windy, cloudy day, that's what this was. Both hit the ball hard. Both covered the court from corner to corner.
"We put on a great show," Azarenka said.
The other semifinal is No. 13 Agnieszka Radwanska of Poland against No. 20 Garbine Muguruza of Spain.
The men's quarter-finals are Wednesday: Novak Djokovic vs. Marin Cilic, Roger Federer vs. Gilles Simon, Andy Murray vs. Vancouver's Vasek Pospisil, and Stan Wawrinka vs. Richard Gasquet. Djokovic finished his 6-7 (6), 6-7 (6), 6-1, 6-4, 7-5 fourth-round victory over Kevin Anderson on Tuesday; it was suspended because of darkness after four sets Monday.
Radwanska, the 2012 runner-up, eliminated No. 21 Madison Keys of the United States 7-6 (3), 3-6, 6-3. Muguruza reached her first major semifinal by defeating No. 15 Timea Bacsinszky of Switzerland 7-5, 6-3.
Radwanska compiled 13 winners, 35 fewer than the big-hitting Keys. But on the flip side was this statistic: Radwanska made seven unforced errors, Keys 40.
Just as Keys pushed Radwanska, the 47th-ranked Vandeweghe gave Sharapova all she could handle, especially in the second set, building a 19-5 edge in winners. Soaking it all in during her first Grand Slam quarter-final — it was Sharapova's 23rd — Vandeweghe repeatedly waved her arms after significant points, motioning to spectators to make more noise and be less, well, genteel.
"I relished it pretty well. I enjoyed my experience. I enjoyed the crowd out there," said Vandeweghe, whose grandfather and uncle were NBA players and grandmother was a Miss America. "I didn't enjoy the result too much."
That's because Sharapova, so passive in the second set, turned it on at the end, claiming the final three games.
Similarly, Williams was too good in the late going. From 2-all in the second set, Williams went about 45 minutes without dropping a game, taking that set and going ahead 3-0 in the third. Azarenka wouldn't concede, and even had a break point in the final game.
Williams erased that with an ace, one of a half-dozen in her final two service games.
"I mean," the 33-year-old American said, "that's my game on grass — just aces."
Oh, but it's so much more.
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