It's now best two out of three for the U.S. Open men's championship.
Down a break in the second set, Juan Martin del Potro came storming back to take the set in a tiebreaker, winning 7-5 after having three match points at 6-3. It looked dicey for an instant after del Potro blew the first match point with an easy overhead that sailed long, but when the Argentine got his serve back he hit a sideline winner.
Federer had been 18-3 in tiebreaks in Grand Slam finals.