Serena Williams has beaten second seed Anett Kontaveit in three sets to advance into the third round of the US Open and delay her expected retirement.

The 40-year-old recently suggested that the final Grand Slam of the year would be the last tournament of her legendary career, and she went into her second-round match as an underdog for many given Kontaveit's lofty ranking.

However, Williams brushed off her shaky form in the build-up to the tournament with a 7-6[4] 2-6 6-2 victory over the Estonian world number two.

The six-time US Open champion battled through an attritional opening set which lasted more than an hour, including an 11-minute seventh game which saw Kontaveit survive three break points before holding serve.

Williams did have the chance to serve for the opening set after breaking in the ninth, but Kontaveit broke back immediately to ultimately force a tie-break which the American won 7-4.

Kontaveit roared back in the second set, racing into a 3-0 lead and breaking Williams in all of her opening three service games, before seeing out the set 6-2.

However, the home favourite earned three breaks of her own in the decider to seal another memorable win on her farewell tour, keeping hopes alive of a record-equalling 24th Grand Slam singles title to end her career.

Despite the expectation that this year's US Open will be her final tournament, Williams stressed that she was in no rush to hang up her racket.

"There is no rush here. There is still a little left in me," she told reporters after the match.

"I'm a pretty good player, this is what I do best. I love a challenge and I'm rising to the challenge. I haven't played many matches but I've been practising really well. Now it's coming together in New York."

Williams, who is ranked 605th in the world and will next face Australia's Ajla Tomljanovic on Friday, has been stuck on 23 Grand Slam singles titles - one short of Margaret Court's overall record - since winning the 2017 Australian Open.