Nineteen minutes later, she finished off a 6-2, 6-0 win Thursday over No. 112-ranked Garbine Muguruza with an ace at 128.7 mph the fastest she can remember serving.
"I'm on the up and up, I feel. It can only get better from here," she said, adding that she wasn't bothered by the ankle during the match. "Obviously when you go out to play you're heavy on adrenaline and you're really pumped up," she said. "Usually I feel injuries after the match but so far, so good. I felt pretty, much better than I ever dreamed of expecting to feel."
Williams said she was bleeding from the lip at one stage, "But it's OK. It's a war wound."
"I think it happens to everyone, but I have never busted it wide open like that," she said. "So, yeah, I was like, 'Oh, no. I can't have a tooth fall out.' That would be horrible."
Defending champion Victoria Azarenka practically danced into Rod Laver Arena for the match before Williams' victory, and said she's starting to find some rhythm after beating Eleni Daniilidou 6-1, 6-0 in 55 minutes.
Also advancing were No. 14 Maria Kirilenko, who beat China's Peng Shuai 7-5, 6-2, No. 16 Roberta Vinci and Elena Vesnina, who beat No. 21-seeded Varvara Lepchenko of the United States 6-4, 6-2.
U.S. Open champion Andy Murray had a 6-2, 6-2, 6-4 victory over Joao Sousa of Portugal. Murray, who ended a 76-year drought by British men in Grand Slam tournaments with his win at Flushing Meadows last year, didn't allow Sousa a single break point chance in steamy conditions.
O Serena Williams closes out her second-round 6-2, 6-0 win over Garbine Muguruza with an ace clocked at 128.7 mph.
• Defending women's champion Victoria Azarenka routs Eleni Daniilidou 6-1, 6-0.
Thursday's TV • Third round, 9 p.m. and 1 a.m., ESPN2