Novak Djokovic beats Alex Zverev to reach Australian Open semifinals and set up clash with Russian record-breaker Aslan Karatsev

World number one Novak Djokovic propelled himself into the Australian Open semifinals by winning a tough four-set battle over Germany’s Alexander Zverev 6-7, 6-2, 6-4, 7-6 - and he will now play Russian sensation Aslan Karatsev.

The defending champion will hope there are no more surprises in store from rank outsider Karatsev, who became the first qualifier since 1968 - known as the Open era of professional tennis - to reach the last four on a Grand Slam debut by beating seed Grigor Dimitrov in his quarterfinal.

Zverev got off to a brilliant start, breaking top seed Djokovic on his first serve to take an early 2-0 lead.

Djokovic’s participation in the match was threatened by a back injury. He tried to return the break but multiple errors throughout the opening set kept him from restoring the balance.

Zverev could have won the first set with break points on Djokovic’s serve, but the world number saved his serve and made a stunning comeback when he most needed it, tying the set at 5-5.

After squandering several set points, the 23-year-old finally got the upper hand in a nail-biting tie-break, prevailing 8-6.

The beginning of the second set mirrored the opener, but this time around it was Djokovic who took the lead, showing no further signs of his injury.

The unstoppable Serb doubled his lead with a four-point advantage that killed the suspense of the set dead.

Zverev’s attempts to cut the deficit fell short as he managed to win just two games in the set, but the German player redeemed himself for that slump by winning three games in a row in the third set, with Djokovic assisting him on one break point by producing a double-fault on his serve.

Djokovic continued to mix splendid shots with inexplicable errors, proving why he is considered one of the best players in the world by delivering yet another comeback to win the set 6-4.

Zverev, as was now something of a tradition in the match, took an early 3-0 lead in the beginning of the fourth set, but again allowed his opponent to regain momentum and make it 3-3.

Both players then played solidly on their service games, sending the set into a nerve-wrecking tie-break. That ultimately determined the winner of the match, as Djokovic avenged his narrow early setback by edging it 8-6 to seal his progress.