It is unusual to picture a 39-year-old who has just completed five hours and 15 minutes on court stepping up to face the world number one in a Wimbledon semi-final.

Yet this is Novak Djokovic, and he remains anything but ordinary in the finest sense.

On Friday, Djokovic will meet world number one and defending champion Jannik Sinner for a spot in Sunday's final.

They clashed at the same stage last year, when Sinner prevailed in straight sets even while dealing with an elbow issue and Djokovic was hampered by a fall in his prior match.

This time Sinner enters without injury concerns and has dropped just one set after his opening-round battle.

Djokovic, however, required a medical timeout after appearing to strain his left calf early in his gripping quarter-final against Felix Auger-Aliassime, then fought until nearly 11pm to overcome the Canadian.

Age, court time and the head-to-head favor Sinner, yet Djokovic defeated him in their most recent meeting at this stage of the Australian Open and remains a threat.

Djokovic is driven by the chance for further history, standing just two victories from an outright record 25th Grand Slam singles title to surpass Margaret Court.

The Serb advanced via walkover and a mid-match retirement in Melbourne but has won only one of his five Wimbledon matches in straight sets here.

"I still try to prove to myself and others that I'm able to compete with the best players in the world and beat them on the biggest stage," Djokovic said.

"That's what I've done in Australia, it's what I've done here.

"Hopefully I can do it for a few more matches in London."

Djokovic may trail Sinner by 15 years, but his movement and shot-making against Auger-Aliassime, even deep in the fifth set, proved remarkable.

He has long shown exceptional durability, as seen at the 2012 Australian Open when he defeated Andy Murray in a four-hour and 50-minute semi-final on Friday, then beat Rafael Nadal in a five-hour and 50-minute final on Sunday.

Fourteen years later, Djokovic entered last year's semi-final against Sinner with limited reserves.

"I do think grass is the least physically demanding surface," former British number one Tim Henman said on BBC TV.

"Those matches over the best-of-five sets are inevitably going to take it out of anyone but, when you're 39, being able to gauge those energy levels is really important."

Djokovic saved 16 of 18 break points against Sinner in the Melbourne semi-finals in January, and his serve has remained reliable at Wimbledon.

Sinner, who leads the head-to-head 6-5, has also served at a high level after adjusting his technique by shortening his ball toss and adding time to his motion.

He has recorded 97 aces against 16 double faults across five matches, including winning 84 percent of first-serve points versus big-server Jan-Lennard Struff in the quarter-finals.

After a five-set scare against Miomir Kecmanovic in round one, Italy's Sinner has settled into the event.

Sinner also handled the heat well against Struff, unlike his difficulties in the warm, humid conditions in Paris last month.

The Italian exited in the second round of the French Open amid extreme heat after noting he "got lucky" when the heat rule applied at the Australian Open during a third-round match at 40C.

He has appeared more at ease as the tournament progressed and, like Djokovic, gained from two rest days after quarter-finals on Wednesday.

"I feel like every match is different [against Novak]," Sinner said.

"Even when I had this small streak with him, I felt like every match has really its own story.

"Especially when you play on a surface like this, if you have a bad serving day or not feeling the ball very well, it's going to be very, very tough."