The top seed was denied a 14th Grand Slam title by in-form Swiss star Stanislas Wawrinka, who was crowned champion with a 6-3 6-2 3-6 6-3 victory.
Nadal, who was still distraught and fighting back tears as his press conference started, said he first felt a twinge during the warm-up, and the problem gradually got worse.
"It was a little bit worse in the first set. End of the first set, I start to feel worse," he said.
He took a medical timeout when 2-1 down in the second set and bravely battled on, but his movement was restricted and the pace of his serves dropped significantly.
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Despite clearly being in pain, the Spaniard never considered calling it quits.
"Last thing that I wanted to do was a retirement. No, I hate to do that, especially in a final," he said.
"Same time, it's tough... During the whole year you are working for a moment like this, and it arrives and you feel that you are not able to play at your best.