A disbelieving Corretja stared briefly, then turned his back to Federer and pulled down his shorts, showing his underwear. Federer was laughing so hard he had to delay the next point.
source: yahoo
Oh, I found the answer. Here's a little more of that yahoo article:
In the Federer-Corretja match, Corretja trailed 4-0 in the second set and hit what appeared a winner. But Federer made a lunging backhand pickup that barely dropped over for a point.
A disbelieving Corretja stared briefly, then turned his back to Federer and pulled down his shorts, showing his underwear. Federer was laughing so hard he had to delay the next point.