She's the world's No. 1 ranked tennis player – he's the greying host of The Late Show.
So no prizes for guessing who won when David Letterman played Serena Williams on a hastily constructed court outside the Ed Sullivan Theatre in New York on Wednesday afternoon.
The 32-year-old showed off her fabulous six-pack tummy in a black cropped top and figure-hugging white skirt with horizontal thin black lines.
