Novak Djokovic smashed his racquet en route to a tense, four-set win over Germany’s Alexander Zverev to reach his 39th Grand Slam semi-final at the Australian Open on Tuesday.

It was a patchy performance from the world No 1, who is nursing an abdominal injury and drew deeply on his mental fortitude to win 6-7 (6/8), 6-2, 6-4, 7-6 (8/6)in three hours and 30 minutes.

Australian Open: Djokovic soldiers on, overcomes Zverev in a slugfest to reach semi-final

“With this condition (injury), I need time to warm up and actually feel like I can rotate well,” said Djokovic, who started sluggishly but finished with 23 aces and 48 winners, along with 56 unforced errors.

“I started moving better and playing better. I served extremely well and I think I had slightly more aces than him, which for me is a miracle.

“There were a lot of nerves out there and I feel emotionally a bit drained.

“I kind of regained my focus after I broke that racquet and things started to shift to me in a positive direction.”

Djokovic will be hot favourite to advance to Sunday’s final when he plays Russian qualifier Aslan Karatsev, who continued his dream run with a 2-6, 6-4, 6-1, 6-2 win against injured Bulgarian Grigor Dimitrov.

“I hadn’t seen him play before the Australian Open,” he said of 114th ranked Karatsev.

“He’s very physically strong and moves well. He has a lot of firepower from the back of the court.

“He’s motivated and has nothing to lose.”

Djokovic had an eventful build-up to the tournament over the quarantine regulations in Australia. Having clarified his stances earlier, he brought up the topic on Tuesday in reference to the injuries we have seen at the tournament.

“There are too many injuries,” added Djokovic, when speaking about Grigor Dimitrov’s struggles against his next opponent Karatsev.

“I’m just hoping that this [quarantine] is temporary, so we could return to preparation without interruptions. The number of injuries show the impact the 14-day quarantine has had on the on the players’ bodies. It’s taken it’s toll unfortunately,” he added.

The Serb star spoke about it in the press conference as well later.

Watch the match highlights and post-match reactions here:

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“My good intentions for my fellow competitors in Melbourne have been misconstrued as being selfish, difficult, and ungrateful. This couldn’t be farther from the truth,” Djokovic had said in a letter he posted on social media.

Full text: Novak Djokovic’s open letter after facing criticism for quarantine demands in Australia

(With AFP inputs)