A defiant Serena Williams bid an emotional goodbye to the U.S. Open with a third-round loss to Ajla Tomljanovic on Friday, in what may have been the last singles match of her glittering career.

Defeat has always been hard to swallow for the fiercely competitive Williams and no doubt the 7-5 6-7 (4) 6-1 loss to the 46th ranked Australian stung her to her core.

But after a joyous run into the third round there was no shame in a loss to the gritty Tomljanovic, allowing the 23-time Grand Slam winner to exit with dignity intact and head held high.

Her three matches, highlighted by a second-round win over world number two Anett Kontaveit, were a gift to her fans, the relentless never surrender attitude that made her tennis' dominant player for over two decades on display right until the very final point.

Always up for a fight, the 40-year-old came out swinging, forcing Tomljanovic to go the distance. The Australian needed six match points to deliver the knockout punch and bring an end to an engrossing three-plus-hour slugfest.

Williams had signaled her intention to retire last month, saying she was "evolving away from tennis" but never confirming the U.S. Open as her final event.