IF ANYONE COULD DO IT ...
Serena Williams can 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.