Novak Djokovic pushed all the way before beating Gilles Simon in five set epic

Djokovic beat Gilles Simon 6-3 6-7(1) 6-4 4-6 6-3, but made over 100 unforced errors in a four and a half hour display that was worryingly shaky for the defending champion.

Up next for Djokovic is Kei Nishikori, the Japanese star who beat the Serb in the US Open semi-finals two years ago.

"Playing against a player like Gilles you can expect a lot of rallies, he's always making you play an extra shot," the Serb said.

"He was fighting, he was playing well, it was physically very demanding and I'm just very glad to get through this one.

"He's a counter-puncher, probably one of the best if not the best on tour ... credit and congratulations to him for a great fight and I wish him all the best."

Having hurtled into the match without dropping a set, Djokovic found a brick wall in the form of the 14th seed Simon whose dogged scrambling frustrated the Serb for much of the four-hour 32-minute clash at Rod Laver Arena.

The world number one racked up a most un-Djokovic-like total of 100 unforced errors and brought the fifth set largely upon himself with a string of botched drop-shots.

But the Serb's iron-willed composure returned quickly in the decider as he broke Simon early and motored to a 5-1 lead.

Simon rallied with a final flourish, regaining a service break and saved two match points before holding to raise thunderous cheers from a raucous crowd firmly in his corner.

But after raising a third match point with an ace, Djokovic sealed the match with an imperious backhand down the line and after shaking hands with his opponent, was glad to slump into his chair.

Djokovic, who booked his 27th successive quarter-final at the grand slams, will now meet seventh seed Nishikori, the man who beat him in the 2014 U.S. Open semi-finals.

"We've played many times in the big tournaments. I lost to him in the semi-finals of the U.S. Open so I hope I'll be able to decrease my unforced error stat ... and do my best in that one."

Part of doing his best will probably involve a change in tactics: time and again Djokovic tried and failed to make a mark by hitting drop shots against the Frenchman.

But time and again he either overhit them, giving Simon the chance to reach the ball and hit back, or else sliced them into the tramlines.

It was something which did not escape the notice of the crowd. "No more drop shots please!" shouted a heckler during the post-match interview - to which Djokovic replied candidly, "I hate to say it, but you know what? You're absolutely right."