In 2nd set, at 4-5 when it was 15 all, Nadal made a return which seemed out and when Federer hit the volley, Nadal made a cross court passing winner. Federer was startled and lost the game and hence the set. Then it showed on replay, that the return was out. If Fed had challenged, he should have won the game. He might have won the 2nd set tiebreak. Match could have been over in 3 sets.