The first signs of vulnerability in Novak Djokovic's Australian Open title defence have appeared in a marathon five-setter but the world No.1 advanced to a quarter-final against Kei Nishikori.

Djokovic capitulated in a second-set tiebreaker against Gilles Simon, the first time he's dropped a set since a semi-final win over Stan Wawrinka at the Paris Masters last November.

The Serbian superstar was also broken and bettered in the fourth set but overcame a staggering 100 unforced errors to triumph 6-3 6-7 (1-7) 6-4 4-6-3 at Rod Laver Arena on Sunday night.

French workhorse Simon made him earn it over the course of four hours and 32 minutes.


Djokovic was sloppy throughout, but still managed to extend his run to 27 consecutive grand slam quarter-finals appearances.

But there were signs of hope for Nishikori.

The 14th-seeded Simon managed to put pressure on Djokovic through a combination of pin-point ball placement, outstanding returns and relentless energy in the pair's many long-running rallies.

Djokovic's false steps and loose strokes didn't help the top seed either.

Djokovic started shakily, scrambling to save seven of eight break points in the opening set.

Further frustration was to come for the five-time Open champion, who stepped up only for his opponent to save 11 of 11 break points in the second set.

That included a 16-point service game, when Djokovic had four chances to take a 5-3 lead in the set but couldn't capitalise.

Simon won a 32-shot rally to take a 3-0 lead in the breaker, powering to square it at one set all.

Djokovic went ahead 2-0 in the third set but still lacked his usual clinical edge.

Simon broke back before Djokovic regained the ascendancy, as the 28-year-old has done on so many occasions in a career that has featured 10 grand slam titles.

However, Djokovic continued to mix dominance with dire as Simon broke late for a 5-4 lead in the fourth set.

Djokovic survived three set points before Simon served it out.

The turning point came when Djokovic broke for a 3-1 lead in the fifth set, fighting back after Simon had skipped 40-0 clear before surging to 5-1 and serving out the match on his second opportunity.

Nishikori earlier vowed to draw on his win over Djokovic in the 2014 US Open semi-final as he steps up a low-key pursuit of a maiden grand slam title.

"I do watch it sometimes when I want some ... confidence," Nishikori said after crushing Jo-Wilfried Tsonga 6-4 6-2 6-4 in two hours.

"That match I played some of the best tennis in my life ... I'm sure I'll try to watch again before next match."

However, Nishikori also recalled being belted 6-1 6-1 by Djokovic two months ago at the ATP World Tour Finals.

"We played in London and he, you know, kind of destroy me," the Japanese star said.

Nishikori totally outclassed former Open finalist Tsonga to become the first man through to the quarter-finals.

The seventh seed has dropped one set in his opening four matches but knows things won't be so easy in the round of eight.

"He (Djokovic) has been playing a lot of good tennis again this year. He doesn't give you easy points, any free points," Nishikori said.

"I have to ... be the one to dictate."