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!" 

Michelle Obama hailed Serena for her rise to super-stardom from the tough Californian neighbourhood 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 unbelievable 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.

Williams prepares for a new life

Williams said she is looking forward to a life beyond tennis after her dreams of a fairytale 24th Grand Slam victory at the US Open ended in defeat.

Williams, who turns 41 later this month, plans to "evolve" away from tennis and is already eyeing her post-tennis life.

"It takes a lot of work to get here. Clearly I'm still capable," Williams said. "It takes a lot more than that. 

"I'm ready to, like, be a mom, explore a different version of Serena.

"Technically in the world I'm still super young, so I want to have a little bit of a life while I'm still walking."

While Williams said she hopes to remain involved in tennis in some capacity, in the near-term she plans to spend more time with daughter Olympia, 5.

"It's been really hard on her, my career," she said. "So it will be nice just to do that and spend some time with her, do things that I never really have done or had an opportunity to do."

Williams admitted however she was uncertain about what role tennis would play in her life. 

"I've had so many amazing moments, that I don't see a future without it," she said. "What's my involvement? I have no idea."

Williams has stopped just short of definitively announcing her retirement, and once again deflected a question about whether there was any "wiggle room" in her apparent desire to hang up her racquet. 

"I'm not thinking about that. I always did love Australia, though," she quipped in a reference to next January's Australian Open, a tournament she has won seven times.

A ferocious three-hour battle

Earlier, Williams had poured everything into one last stand in front of a ferociously partisan crowd at the Arthur Ashe Stadium during her three-hour battle with Tomljanovic.

After levelling the match in a marathon 1hr 23min second set, the veteran's resistance was finally broken in the third set as Tomljanovic raced into a 5-1 lead.

Even then, Williams summoned every last drop of her fighting spirit in an attempt to stave off defeat, saving five match points in the seventh game before finally succumbing.

Afterwards, Williams saluted the crowd, welling up as Tina Turner's anthem "Simply the Best" belted out around the stands.

"It's been a fun ride," Williams said. "It's been the most incredible ride and journey I've ever been on, I mean, in my life. 

"I'm just so grateful to every single person that's ever said, 'Go, Serena,' in their life. You got me here."

Williams reserved special praise for her parents Richard Williams and Oracene Price who put her and elder sister Venus on the path to tennis superstardom.

"It all started with my parents," Williams said. "They deserve everything."