Alcaraz had breezed through the Melbourne draw to the last eight, going through the gears at the kind of pace seemingly needed for a tilt at the title.

But he came unstuck as Zverev - who knocked out Britain's Cameron Norrie in the previous round in a gruelling five-setter - produced his very best level in an inspired start.

Zverev used his huge first serve and heavy groundstrokes to devastating effect in a one-sided opening set.

Alcaraz won just two receiving points in the opener, and made 11 unforced errors, as Zverev took the lead in just 29 minutes.

With the majority of the crowd seemingly behind the Spaniard, a sense of shock swept around Rod Laver Arena.

It continued to linger as Zverev maintained a sky-high level, demonstrating the talent which has seen him long predicted to win one of the sport's major prizes.

Midway through the second set, Alcaraz finally started to find his range with the forehand before Zverev fought off two break points for 3-3.

But any thoughts from the Spanish fans it could be the start of a longer revival proved unfounded as Zverev won four games to a row for a two-set lead.

Zverev continued to apply pressure in the third set, moving 3-1 ahead when Alcaraz handed over another break with a double fault and a backhand long from deuce.

Serving for victory, Zverev dug himself out of trouble at 0-30 with two enormous first serves but Alcaraz - knowing it was now or never - responded again to break for the first time.

Suddenly Alcaraz had momentum. With his belief increasing, he walloped a series of stinging winners in the tie-break and provided hope to his vocal supporters.

Zverev received treatment on a foot issue before the fourth set and many of the thousands who had stayed on Rod Laver Arena beyond midnight might have envisaged seeing the contest go into a decider.

But Zverev ensured that did not happen. He broke for 5-4, helped by Alcaraz making three loose errors with his backhand, and did not miss a first serve as he wrapped up victory.

"When you're up 6-1 6-3 5-2, you start thinking. I mean we're all human. It's not helpful," said Zverev.

"I fought back quite well in the fourth set, didn't let go and [was] very happy to finish the match."