The world No 3 was hampered by cramp, forcing him to call for the doctor, but the Spaniard dug deep for a 6-2 3-6 6-7 6-3 7-5 win in the second round at Melbourne.
Following a routine opening set, Nadal started to wilt in the humid conditions and the American gained a foothold by levelling in the second.
The usually sprightly Spanish player was showing signs of fatigue in the third set, taking his time between changeovers and wearily dished up a double fault while serving for the third set at 5-4.
Smyczek appeared to sense his opponent's vulnerability, taking the resulting tie-break with an ace, but Nadal produced a determined response to force a deciding set.
The world No112 continued to drag out the gruelling encounter, saving break points in the first and seventh games, before Nadal finally made the breakthrough for a 6-5 lead.
There was yet more drama as a spectator shouted out before the left-hander served a fault and Smyczek showedfine sportsmanship by allowing the point to be replayed.
Nadal won it to claim three match points, but was hauled back to deuce before eventually securing his progress in four hours and 12 minutes.
"It was a very tough night for me," said Nadal. "Iwant to congratulate Tim because he was a real gentleman what he did in the lastgame. Not a lot of people would do something like this at 6-5 in the fifth. Ithink he played a great match.
"It was a little bit humid today. I felt very tired after the first set andthrough the whole match.
"I don't know what was going on. I hadsome cramps around the body in different places so that says I was not in perfect shape. It's not normal after an hour."