The 31-year-old said she felt pain and swelling and may need an X-ray, but pledged to soldier on in her quest for a sixth title at the year’s first Grand Slam.
“Oh, I’ll be out there. I mean, unless something fatal happens to me, there’s no way I’m not going to be competing,” she said. “I’m alive. My heart’s beating. I’ll be fine.”
Any lasting injury to the hot favorite could throw the women’s draw wide open and may benefit defending champion Victoria Azarenka, who is scheduled to meet Williams in the semi-finals.
The world No. 1 from Belarus beat Romania’s Monica Niculescu, but had to come from 3-0 down in the second set before winning 6-1, 6-4.
Caroline Wozniacki, last year’s top seed and now ranked at 10, also had to dig deep before reeling off six third-set games in a row to beat Germany’s Sabine Lisicki.
Elsewhere, Japanese No. 1 Kimiko Date-Krumm, aged 42, became the oldest female to win a main draw match at the Australian Open, upsetting 12th seed Nadia Pedrova 6-2, 6-0.
In the men’s draw, Murray, in his first Grand Slam match since becoming a major-winner, showed his class in a 6-3, 6-1, 6-3 dismantling of Dutchman Robin Haase.
Federer, who is drawn to face Murray in the semi-finals, also showed little mercy in his 6-2, 6-4, 6-1 win over Frenchman Benoit Paire and faces Nikolay Davydenko.