While making an appearance on 'Late Night Show' with David Letterman, Serena Williams got all dolled up for the occasion, which get Letterman's attention right away.

Maybe Letterman has a rule that says his female guests have to dress sexy, because it's not the first time that ladies have donned a sexy outfit, and rendered Dave speechless.

'Glee' star Lea Michelle made Letterman's jaw drop earlier this year, and Eva Longoria did the same thing last year. Perhaps Letterman has it in their dressing room that if they want more camera time, they need to impress him. Dave, you old dog, you.