“It gets easier and easier with opponents and their serves,” Djokovic joked in an on-court interview with four-time grand slam winner Jim Courier. “Milos has been playing the tennis of his life in the last 15 months. We are good friends and practice a lot together. He is one of the new generation of stars.”

A single break of serve was enough for Djokovic to win each set against Muller, although the top seed had to save four break points midway through the third set and appeared unhappy with the lack of grip from his footwear on several occasions.

“Not playing Gilles before it was tricky coming in,” Djokovic added. “I did not know what to expect. I knew he had struggled with injuries recently but he beat some top players here and deserved a lot of respect.

“I was fortunate to serve very well in important moments in the third set. It was very hard to read his serve and he has a lot of variety in his game.”

Roger Federer was surprisingly the only top seed not to advance to the last eight, the 17-time grand slam winner having lost in the third round, with fourth seed Stan Wawrinka, fifth seed Kei Nishikori and eighth seed Raonic all winning on Monday.

Defending champion Wawrinka looked to be cruising to victory over Guillermo Garcia-Lopez when he led by two sets to love and 4-2 in the third, only to lose the next four games in succession and trail 5-0 and 6-2 in a fourth-set tie-break before clinching a 7-6 (7/2) 6-4 4-6 7-6 (10/8) victory.

“It was a real battle today,” Wawrinka said. “I was two sets up with a break and in the end I’m happy to get through. There was a good level of tennis in general and it’s good to make one more win.

“At this level, things can change quickly. I did maybe two mistakes, then he start to play a little bit faster, a little bit more flat. I started to be defensive a little bit too much.

“Losing 5-0 was a bad start to the tie-break. At 6-2 I knew it was close to come back because I had the wind with me. I had to focus on every point. I knew if I was going to get it back to 6-5, he was going to get nervous. I did a good passing shot along the line and just focused point after point.

“I’m happy to get through. It is great for me to be in quarter-final and still there. I’m not focused on winning (another) major. I know it’s difficult to do it. It’s amazing I already did one. I’m taking it match after match.”

The match was played on Margaret Court Arena after it was officially re-opened by the eponymous 24-time grand slam singles champion, although play was embarrassingly delayed for several minutes in the first set due to water dripping onto the court from the new retractable roof.

Wawrinka will face Nishikori in the last eight after the US Open finalist beat Spain’s David Ferrer 6-3 6-3 6-3.

Eighth seed Raonic squandered two match points in the fourth set tie-break but regained his composure to secure a 6-4 4-6 6-3 6-7 (7/9) 6-3 victory over Spain’s Feliciano Lopez.