Watching close friends and fellow Americans Tommy Paul, Taylor Fritz, Frances Tiafoe and Ben Shelton thrive in his absence helped keep him motivated.

“I watched Novak become the greatest in that two-year period off. You find yourself asking yourself, ‘What would Novak be doing in my situation?’,” Opelka said.

“Even when you’re in a sleeve, you’re always trying to kind of maximise, and that’s the effect he’s had on the sport. I think that’s why you see a guy like Jannik, who reminds me a little bit of someone else 10 years ago, and Carlos [Alcaraz] and stuff.

“I stayed the course, even when I was in a cast and on crutches, and was just hoping to have another chance out here like tonight.”

Opelka, who will meet fellow giant Giovanni Mpetshi Perricard in the semi-finals, remained deadpan in victory, much like he was throughout his upset win.

He achieved it with first-strike tennis, unafraid to swing hard and early to avoid elongated rallies that would have played into Djokovic’s hands.

One game after staving off the sole break point he faced, Opelka blasted through Djokovic’s defences to snatch a 3-1 lead in the second set – and he never looked like handing it back from there.

“Obviously, this is great, [but] this is all prep for Melbourne,” he said.

“I’m excited, but it’s the beginning of a new start for me. I’ve got a long way to go to get my ranking back where I would like it, and my endurance back where I would like it, to be able to compete at this level week in, week out.

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“Anyone can do it for a night or two, here and there, but the difficult part is to sustain it over an 11-month season.”

The other men’s semi-final is between defending champion Grigor Dimitrov and Czech Jiri Lehecka.

The women’s final four pits top seed Aryna Sabalenka against Russian rising star Mirra Andreeva, and qualifier Polina Kudermetova takes on Ukraine’s Anhelina Kalinina, who ousted Gold Coast’s Kim Birrell in three sets.