WATCH | Serena bids emotional farewell as Tiger, Michelle Obama lead tributes to 'the greatest'

Tiger Woods and former first lady Michelle Obama hailed Serena Williams as "the greatest" on Friday as the tennis superstar headed into retirement.

Williams, the winner of 23 Grand Slam titles, played what was almost certainly the last match of her 27-year career at the US Open when she slipped to a three-set defeat to Ajla Tomljanovic.

"@serenawilliams you're literally the greatest on and off the court," tweeted Woods, a 15-time golf major winner who was in Serena's player box for her second round win on Wednesday.

"Thank you for inspiring all of us to pursue our dreams. I love you little sis!"

.@serenawilliams you’re literally the greatest on and off the court. Thank you for inspiring all of us to pursue our dreams. I love you little sis!!!!!!

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Williams bade an emotional farewell after her match on the court after her defeat.

"I wouldn't be Serena if there was no Venus," she said, giving flowers to her older sister and fellow multiple grand slam winner. 

??? "Venus is the only reason Serena Williams ever existed!"An emotional Serena Williams bids farewell to the #USOpen after losing to Ajla Tomljanovic.

Michelle Obama hailed Serena for her rise to super-stardom from the tough Californian neighborhood of Compton to sporting and cultural icon.

"Congrats on an amazing career, @SerenaWilliams! How lucky were we to be able to watch a young girl from Compton grow up to become one of the greatest athletes of all time," tweeted Obama.

"I'm proud of you, my friend and I can't wait to see the lives you continue to transform with your talents."

Thank you @serenawilliams! You’ve changed the game of tennis forever. Through all the different phases of your career, your love