Rafael Nadal hadn't dropped a set to Stanislas Wawrinka before today's Australian Open final. Wawrinka promptly changed that by winning the first set, eventually taking the match in four sets, 6-3, 6-2, 3-6, 6-3. But Wawrinka didn't come out on top only because of Nadal's injury. The Swiss was in charge the entire time.
It just became more favorable for Wawrinka when Nadal's back flared up during the second set. He used an injury timeout to try and recover, but it clearly affected his play. Nadal was slower at getting to shots, and his serve speed decreased drastically. No one had retired from a men's Australian Open final since 1990, and Nadal didn't want to break that streak. Bowing out in a final would be such an anticlimactic finish to the tournament.
And so, Nadal stayed, and actually won the third set, but Wawrinka ended it mercifully in in the fourth. Wawrinka earned his first Grand Slam title while Nadal experienced pain without the glory.