Serena Williams ranks alongside such icons as Muhammad Ali among the greatest legends in sport, retired US tennis stars John McEnroe and Chris Evert said on Wednesday.

Speaking ahead of their ESPN commentary duties for the US Open that begins on Monday, Evert hailed 23-time Grand Slam champion Williams, who is likely to retire after this year's Grand Slam showdown in New York.

"I would put her up there as far as the influence, the fact how she revolutionized the game and also how she's influenced girls and women and people throughout the world," Evert said.

"That's how big of a star. She's a superstar. What impresses me the most is just off-court influence and how she's influenced young women to own their power, to speak their mind, to be fearless."

McEnroe gave Serena the GOAT label -- as in Greatest of All Time.

""All you need to say about Serena is that she's put herself in that pantheon of GOATs -- Billie Jean King. Muhammad Ali, Michael Jordan, Tom Brady. That's where Serena is.

"She has become like an icon of icons. It probably happened later than it deserved to for her, but it's here now."

McEnroe said Williams changed the game with her power and devastating serve, comparing her to NBA 3-point sharpshooter Stephen Curry.

"Serena took it to that next level because she had the greatest serve ever, better than a lot of guys," McEnroe said. "I would compare her in a way, the way she changed it, to Steph Curry. Everyone is shooting 3-pointers, but no one does it as well as him.

Evert spoke of Williams as an inspiration to others.

"She has been very inspirational off the court," Evert said. "On the court, it's obvious, her record, her power, her mental toughness. But the intangibles, the fearlessness in her, has really impressed me.

"She has never set any limits in tennis or in life. To get that message across to everybody, I think, is very powerful. And she really has spoken her mind... and she has so many platforms, from the body shaming, to working moms, to women of color, and just empowering women.

"That message, off the court, to me and maybe to millions of people, is more profound and more powerful than even what she's done on the court."

Serena changed tennis and attitudes, Evert said.

"She revolutionized tennis. She revolutionized the power in the game," Evert said. "And I feel like she really inspired women of color, because we've seen a lot more women of color playing the game."

When it came to this year's US Open, Evert warned "don't underestimate her" but admitted, "it's going to be tough for her to get to that second week" while McEnroe said, "anything could happen. It's so wide open."

When it came to retirement, McEnroe saw Serena closer to tennis than Evert, who cited Williams having said she wants to have another child.

"I think that she'll be more of an ambassador," McEnroe said. "She sounds like she's got a ton of interests and I'm sure that some of those are going to have to do with tennis."

Added Evert: "I don't envision her really around tournaments... I don't think she's going to be as visible in tennis as she is maybe outside of tennis."