Eight-time winner 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 number one, 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. "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."