I'm still the best, insists Serena ahead of Australian Open

Williams' comments came after her quarter-final win at the Yarra Valley Classic where she looked in ominous form -- before pulling out of the tournament with a right shoulder injury.

The tennis icon, who is aiming for a record-equalling 24th Grand Slam title, said her advancing age was inconsequential and said she looked up to athletes like Brady, 43, who on Sunday is seeking his seventh Super Bowl title. 

"I feel good. I look at Tom Brady, it's so inspiring," she said. 

"I think with all the new things, all the new technology, all the new physical treatments, rehab, things that you can do ... obviously it prolongs careers for so long."

Williams, who defeated fellow American Danielle Collins 6-2, 4-6, 10-6 after a mid-match wobble, was set for a semi-final showdown with world number one Ashleigh Barty Saturday before the injury. 

She had looked sharp having worked her way back from an Achilles injury suffered at last year's US Open, which then led to a withdrawal from the French Open before the second round. 

Williams had moved well and appeared physically strong in her match against Collins, which she won in 96 minutes. 

Asked whether she is still the best in the world when on top form, Williams was unequivocal.

"I absolutely wouldn't be playing if I didn't think that," said the American.    

Williams is drawn to play Germany's Laura Siegemund in the opening round of the Australian Open starting on Monday.