MELBOURNE: Serena Williams continues her long quest for a record-equalling 24th Grand Slam when the coronavirus-delayed 2021 Australian Open begins on Monday, four years since her last major triumph in Melbourne.

It may be this season or never for Williams, who turns 40 in September and will have to conquer a field bursting with young talent headed by US Open champion Naomi Osaka, defending champion Sofia Kenin and the home favourite, world number one Ash Barty.

The tournament was put back three weeks to allow all players to quarantine for 14 days, but a coronavirus case at a hotel saw all play shelved Thursday in warm-up events with hundreds of players and officials ordered back into isolation for Covid tests.

Williams says she had benefited from the delays, using them to recover fully from the nagging Achilles injury sustained during her run to the semi-finals at the US Open, which subsequently caused her to withdraw from Roland Garros.

"I needed time. I don't think I would have been here if it was during the regular season," she said this week.

"I definitely was pushing the limits, but I'm here."

Williams won her 23rd Grand Slam singles title at the Australian Open in January 2017, then revealed she had been pregnant with daughter Olympia.

She has reached four major finals since, twice at Wimbledon and twice at the US Open, but left empty-handed each time and coronavirus-disrupted 2020 was the first year since 2006 that she failed to reach a Slam final.

Osaka comes into Melbourne as US Open champion, as she did two years ago before battling past Petra Kvitova in an epic final to lift the Australian Open's Daphne Akhurst Cup.