The Spaniard revealed that he was in "unbelievable pain" on the eve of the tournament when he heard a crack in his right knee.
Nadal also praised his 33-year-old opponent.
"He's a fantastic player. He's had a lot of injuries the last couple of years but it's always great to play Tommy," he said.
"He's thinking about retiring but I thought he played well this afternoon."
Nadal took the Australian Open title in 2009.