Tiger, Michelle Obama lead tributes to 'the greatest' Serena

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!"

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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."

NBA superstar LeBron James described Serena as a GOAT (Greatest Of All Time).

"Congratulations to you, you had an unbelievabe career," James said in a video message.

"What you have done for sport, for women is unprecedented, it's been an honour to watch your journey and conquer all the goals you set out to do.

"Win, lose or draw, we knew you were the greatest."

Fellow NBA legend Magic Johnson saluted Serena for her inspiring other Black athletes.

"We just witnessed the last US Open for the greatest of all time, Serena Williams!! Serena has meant so much to sports, the game of tennis, the world, every little girl, and even more to every little Black girl across the globe," tweeted Johnson.

Television and entrepreneur Oprah Winfrey described Williams as a "Shero. Legend forever" while US sprinter Allyson Felix said her achievements "will never be lost on me".

Teenage tennis star Coco Gauff, who is widely seen as Williams's heir apparent, hailed her compatriot for inspiring her to take up the sport.

"Serena, THANK YOU. It is because of you I believe in this dream. The impact you've had on me goes beyond any words that can be put together and for that I say thank you, thank you, thank you, GOAT!"

Gauff was the French Open runner-up this year and is already into the fourth round of the US Open.

Andy Roddick, the last American man to win a Slam title in New York in 2003, said watching her play was the "pleasure of a lifetime."

"Can't wait to see what you do next. Thank you my old friend," he tweeted.