The four-time Olympic gold medallist and 23-time Grand Slam singles winner is widely believed to have played her last competitive match after losing to Australia's Ajla Tomljanović in the third round of the US Open on Friday (September 2).

Twitter reported that Williams is the most-tweeted-about female athlete of all time following her US Open exit.

Twitter launched in 2006, at which point Williams had already won seven Grand Slams singles crowns and a women's doubles Olympic gold medal with her sister Venus at Sydney 2000.

Of the users who tweeted about Williams from the start of last month until Friday, 74 per cent had not previously posted about her throughout the year in one statistic provided by Twitter, as reported by American broadcaster NBC.

At the start of this year's US Open, Twitter unveiled a special "GOAT emoji" that appears in tweets including the hashtags #Serena, #SerenaWilliams and #ThankYouSerena.

After her exit from the tournament, it declared that "on the court and on the timeline, [Williams] is the greatest of all time".

Williams' compatriots Simone Biles, a seven-time Olympic gymnastics medallist, and National Football League star Tom Brady, as well as footballers Cristiano Ronaldo of Portugal and Lionel Messi of Argentina, have previously received a dedicated GOAT "hashmoji" from Twitter.

GOAT is an acronym for greatest of all time.

The US Open's all-time single-day attendance record of 72,039 was set at the Billie Jean King National Tennis Center on Friday, including 23,859 spectators for Williams' match against Tomljanović on the Arthur Ashe Stadium court.

Williams turns 41 later this month.