The defending champion found his best touch and was rarely troubled by the 14th-seeded Argentine during Sunday's clash on Rod Laver Arena, advancing 6-3 6-4 6-4.

Gunning for an eighth crown at Melbourne Park, the second seed is locked in for a final-eight showdown with resurgent Canadian Milos Raonic.

Djokovic said he was feeling more confident as the tournament progressed.

"Diego is a great quality player, he's had a terrific tournament and hadn't dropped a set. I knew if I gave him time he could do a lot of damage from the back of the court," he said.

"I kept things pretty much in control in all three sets. Maybe could have finished the match a bit earlier."

Earlier, Raonic was the first player through to the men's quarter-finals when he accounted for 2018 finalist Marin Cilic 6-4 6-3 7-5 on Margaret Court Arena.

No.32 seed Raonic had upset world No.6 Stefanos Tsitsipas in the third round and he continued that impressive form.

Cilic, beaten in five sets by Roger Federer in the 2018 Australian Open final, was unable to convert any of his four break points, with Raonic slamming down 35 aces to his five.

The Canadian served himself out of trouble at a pivotal stage of the third set after Cilic had earned two set points.

Raonic boomed down three aces in succession followed by a forehand winner to level at 5-5, before breaking the Croatian for the third time and then serving out the match.

He has reached the quarter-final stage at the season-opening grand slam five times, with his best result a semi-final appearance in 2016 when he lost to Andy Murray in five sets.

Djokovic said Raonic's serve was always tough to combat.

"I feel like Raonic moves better than (John) Isner and (Ivo) Karlovic. I mean, he's not as tall as these two guys," he said.

Obviously it's a huge advantage when you hit serves from that height.

After reaching the quarter-finals last year at Melbourne Park, a knee problem forced Raonic out of the entire clay court season, including the French Open.

He was unable to play the US Open due to a hamstring issue and was forced out of the Davis Cup final against Spain with a back strain.

"It's always been very positive for me here and I've always found a way to play well," Raonic said.

"I'm fresh physically and mentally at the start of the year and having some time to train in the off-season, it does me a lot of good."

Unseeded American Tennys Sandgren, meanwhile, is through to the last eight in Melbourne for the second time in three years after eliminating Italian world No.12 Fabio Fognini 7-6 (7-5) 7-5 6-7 (2-7) 6-4.

Australian Associated Press