Serena Williams of the US celebrates a point against Victoria Azarenka of Belarus during their match at the US Open tennis tournament in New York, September 3, 2011.
Williams battered the fourth-seeded Belarussian in the first set but then was pushed to a scintillating tie-break to decide a dynamic second set before advancing to the round of 16 at Flushing Meadows with a 6-1 7-6 win.
Firing on all cylinders, Williams roared out to a 5-0 lead in just 17 minutes against Azarenka, a two-time tournament winner this season who reached the semi-finals at Wimbledon and is ranked fifth in the world.
"I think I played really well," said Williams. "I was doing the right things.
"It's a good feeling, for sure. It feels amazing."
The 22-year-old Azarenka described it differently.
"o have somebody just going at you like that, it's a little bit painful," she said.
"You try to do your best, but somebody's on fire."
Williams, seeded 28th after a long layoff due to injury dropped her from the world number one ranking, looked set to turn the tough match-up into a rout but Azarenka bounced back with some of her best tennis.
She changed the rhythm by playing aggressively and the pair produced the best tennis of the women's tournament so far.
Azarenka used deep groundstrokes to force Williams to race from corner to corner, but the American met the challenge, going into a full split after reaching for one forehand, and nearly crashing into a photographers' well after chasing down another shot into the corner.
Williams looked set to finally subdue her opponent when she took a 0-40 lead with Azarenka serving in the ninth game trailing 5-3 and trying to stay alive. The Belarussian fought off three break points in a row and held serve to make it 5-4.
With Williams serving for the match, Azarenka saved another match point in the next game, and put the set back on serve with a reflex block of a forehand smash to break serve for 5-5.
It went to a tie-break and the decider was 5-5 when Azarenka, a semi-finalist at Wimbledon this year, netted a forehand to give Williams her fifth match point.
Williams did not squander this one, inducing Azarenka to send a forehand wide to claim the tie-break 7-5.
"I think the whole match was (the quality of) a semi-final," Williams said.
"At the end of the day, it was a third round. I'm only in the round of 16 now. So, you know, I got to keep going the best I can."
The 29-year-old Williams will next face former French Open champion Ana Ivanovic, the 16th seed, who beat American wildcard Sloane Stephens.
Serena Williams of the US serves to Victoria Azarenka of Belarus during their match at the US Open tennis tournament in New York, September 3, 2011.