In an often breathtaking clash laden with sumptuous shot-making, the Spaniard was rocked by the hard-hitting Schwartzman who broke him three times to claim the second set at Rod Laver Arena.
But the 16-times grand slam champion responded with typical grit, bolstering his defence to blanket the 24th seed's firepower and counter-punching brilliantly to book a match with Croatia's Marin Cilic.
After three cakewalks in the previous rounds, the three-hour 51-minute work-out was timely, said Nadal, who came into the tournament with some queries over a knee injury.
"You can't expect easy matches when you're playing in big tournaments," the world number one told reporters after saving all his seven break points in the final set.
"It's always better winning in two hours than in four. But that's it. It was a good test for me. It was a lot of hours on court. Moments under pressure.
"A lot of positive things that I managed well.
"But being honest, too, moments like this helps to be more confident in yourself, in your body."
The victory ensured Nadal, who lost a five-set classic to long-time rival Roger Federer in the final last year, will retain his world number one ranking.
It also put the Mallorcan equal second with Stefan Edberg on the number of quarter-final appearances in the Australian grand slam in the professional era. Federer, naturally, leads with 14.
The jet-heeled Schwartzman joined the grand slam quarter-final club at the U.S. Open, a milestone moment during an impressive rise up the rankings in 2017.
Nadal had seen off his first three opponents without dropping a set but it was soon clear at Rod Laver Arena that Schwartzman was cut from a different cloth.
TOP SPIN BOMBS
The 1.70m (5-ft 7-in) Argentine virtually needed a step-ladder to reach the bounce of Nadal's top-spin bombs but he was all over the Spaniard's serve from the get-go.
He grudgingly surrendered his own first, though, allowing Nadal to wrap up the first set in 45 minutes.
From there the match went stratospheric in quality, each player landing blows like bare-knuckled prize-fighters.
After six breaks of serve and a feast of glorious shot-making, the fireworks continued in a frenzied tiebreak.
Schwartzman cracked a blazing forehand down the line to lead 2-0, then outpointed the Spaniard with a backhand volley to move within two points of the set.
The pressure told as a forehand ballooned off the frame of Nadal's racquet and another backhand sailed long to put the match on level terms.
Schwartzman was pumped, but he left the court immediately, his extended break allowing Nadal time to marshal his forces.
The lefthander resumed with renewed vigour to break Schwartzman to love and after roaring to 5-2, he claimed the third set with an ace.
The momentum was now all with Nadal. He broke again quickly, leaving Schwartzman to curse loudly in Spanish and bicker with the chair umpire at the change of ends.
The 25-year-old kept swinging for the fences to the finish and saved a second match point by ripping a backhand winner down the line.
Nadal simply bided his time before pouncing on the third, a second serve dismissed with a sizzling return winner to keep his bid for a 17th grand slam title firmly on track.
(Editing by Sudipto Ganguly)
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