AFP,  MELBOURNE: Rafael Nadal says it's time to step up at the Australian Open after fighting off tenacious Argentine Diego Schwartzman to win through to the quarter-finals on Sunday.

The 2009 winner was taken to four physically-demanding sets in almost four hours before prevailing 6-3, 6-7 (4/7), 6-3, 6-3 on Rod Laver Arena, ensuring he remains world number one after the tournament ends.

It will be his 10th Australian Open quarter-final appearance, second only to Roger Federer, and he will face Croatia's sixth seed Marin Cilic. Nadal leads 5-1 in their previous meetings.

It was just the physical examination Nadal was looking for after coming into the year's opening Grand Slam without an official match following knee concerns.

It was the Spaniard's fourth win over the Argentine but his toughest after beating him in straight sets in the second round at the 2015 US Open.

Nadal broke in the eighth game on the way to taking the opening set, but he encountered stiff resistance in the second set.

There were six service breaks as Nadal could not shake off the tigerish Argentine, who levelled the match by winning the tiebreaker when the top seed's forehand was long.

There was an exhilarating moment when Schwartzman ran down a drop shot and won the point with an angled volley across the net to break back at 4-4.

Nadal took control in the third set with a break in the fourth game, but he had to fight off five break points before holding serve in a titanic 13-minute second game in the final set.

He broke in the third game of the fourth set, but had to make some monumental holds of service to keep Schwartzman at bay, winning on his third match point with a return winner.