With del Potro leading 4-2 in the fourth set tiebreak, a fan yelled "out" on a Juan Martin del Potro forehand. Federer stopped playing and then protested to the chair ump. After about 20 seconds, Federer challenged the call and, amazingly, won. The point was replayed, with del Potro winning again.
The irony of waiting 20 seconds to challenge a call seemed to be lost on Roger, but not del Potro.