Williams – who is weeks away from turning 41 – announced last month she was “evolving away from tennis” after more than two decades dominating the women’s game.
WATCH THE VIDEO ABOVE: Serena takes final bow after Aussie’s US Open stunner.
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The epic encounter lasted three hours and five minutes with Tomljanovic defeating the 23-times grand slam champion on Friday night.
Fans, friends, celebrities and competitors have shared emotional tributes to Williams’ final showdown.
“Her incredible career made its mark on tennis history. And yet her greatest contributions may be yet to come. Thank you, @serenawilliams. Your journey continues,” former world number one Billie Jean King said.
Tomljanovic said she was in awe of her rival.
“I’m feeling really sorry just because I love Serena just as much as you guys do. What she’s done for me, for the sport of tennis, is incredible,” she said.
“I never thought I’d have a chance to play her in her last match when I remember watching her in all those finals, so this is a surreal moment for me.
“She’s the greatest of all time. Period.”
Other sports-stars chimed in, including four-time NBA champion, LeBron James.
“You’re a GOAT. What you’ve done for the sport, what you’ve done for women … is unprecedented,” James remarked.
“It was an honour to watch your journey, to watch you conquer all the goals you ever set, to see you break records, to see you just be amazing on the tennis court and also off it.”
Former world number one, Andy Roddick, said: “I love you @serenawilliams. It’s been the pleasure of a lifetime to watch you become what you have. Can’t wait to see what you do next. Thank you my old friend.”
And Michelle Obama took to Twitter to express her admiration for the sporting legend.
“Congrats on an amazing career @SerenaWilliams!” she wrote.
“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.
“I’m proud of you, my friend – and I can’t wait to see the lives you continue to transform with your talents.
Seven-time grand slam champion, Mats Wilander, also had heartfelt worlds for Williams.
“Serena has meant so much for tennis and the world, but for America as well,” he said.
“I was watching the crowd and there are so many people here that have never been to a tennis match before … She has brought tennis to a completely new level and transcended the sport.
“We are going to remember Serena the person forever.”
– With AAP