The 28-year-old had romped through the first set but was then struck by something that almost made him vomit on court and left him sweating profusely.
"I was close to not continuing. I was dizzy, I felt that I might fall down," Nadal told reporters after the 6-2 3-6 6-7(2) 6-3 7-5 victory over the American.
"At the end of the first set, I start to feel my body very bad, very tired. I don't know. I was worrying crazy.
"I was suffering a lot. Too much. (It) was not funny today."
Nadal's problems may have begun in the second set but it was not until early in the third that he sought medical treatment and Smyczek seized his opportunity to canter through the tiebreak.
As fears mounted around Rod Laver Arena that the third seed could suffer an early exit, the man who has a 16-5 record in five-set marathons battled back to open a 5-2 lead in the fourth set before dragging Smyczek into a decider.
The fifth set went on serve until Nadal finally broke the resistance of Smyczek, whose scrambling game resembles that of former top-five player David Ferrer, in the 11th game before wrapping up the win on his fourth match point.
"It was pretty clear Rafa didn't have his best stuff," a magnanimous Smyczek said.
"But it just shows the kind of player, the kind of champion he is because he was sick and not playing well.
"That was his C or D game. He found a way to win. So hats off to him. That's why he's one of the best."
Nadal, who had admitted before the start of the tournament that he was probably not ready for a tilt at the title after being absent for several months in 2014 with injury and illness, took some positives from the win.
"All during my career is obvious that I was able to find solutions for tough moments," said the Spaniard, who set up a third round clash with Israel's Dudi Sela.
"I was able to win matches where I was in trouble (so) I feel lucky to have the chance to finish the match, and then to find a way to win."
(Reporting by Greg Stutchbury, editing by Pritha Sarkar)
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