Serena Williamscan absolutely close out her career in fairytale fashion -- so says ex-tennis star Tommy Haas, who tells TMZ Sports he believes she's no longer the underdog in the U.S. Open.
Williams -- considered the greatest women's tennis player of all time -- is calling it a career at the conclusion of this month's major tournament in New York ... and most didn't give her much of a chance to go out with a win.
"She's looking very good," the one-time No. 2-ranked men's tennis player in the world said. "I think now it's a matter of if she's feeling good physically. I mean, obviously, she's feeling great mentally."
Haas also spoke with us about the court advantage the 40-year-old has received -- everyone from Tiger Woods to Bill Clinton to Zendaya has showed up to cheer her on -- and he noted it's all going to be tough for her opponents to overcome.