You wouldn't normally expect a 39-year-old who had just spent five hours and 15 minutes on court to challenge the world number one in a Wimbledon semi-final.

But this is Novak Djokovic we're talking about - and he is, in the best way possible, not normal.

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

They met at this stage last year, with Sinner winning in straight sets despite having an elbow injury and Djokovic struggling after a fall in his previous round.

This time around, Sinner is injury-free and has not dropped a set after his opening match went the distance.

However, Djokovic needed a medical timeout after tweaking what appeared to be his left calf early in his epic quarter-final against Felix Auger-Aliassime - and then ended up battling until almost 11pm at night to see off the Canadian.

Age, time on court and the head-to-head are all in Sinner's favour, but Djokovic won their last meeting at this stage of the Australian Open and cannot be counted out.

Djokovic is also motivated as he chases more history - he is just two wins away from an outright record 25th Grand Slam singles title, breaking the tie with Margaret Court.

The Serb benefited from a walkover and a mid-match retirement on his way to the last four in Melbourne, but goes into Friday's meeting having won just one of his five Wimbledon matches in straight sets.

"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 in 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 be 15 years older than Sinner but his movement and shot-making ability against Auger-Aliassime, even deep into the fifth set, was astonishing.

He has always had amazing durability. At the 2012 Australian Open, he beat Andy Murray on Friday in a semi-final lasting four hours and 50 minutes, had one day off and then beat Rafael Nadal on Sunday in five hours and 50 minutes - the longest men's singles final in history.

Fourteen years have passed since then, however, and Djokovic went into last year's semi-final against Sinner with "the tank half empty".

"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 the 18 break points he faced against Sinner in the Melbourne semi-finals in January, and his serve has been strong at the championships.

Sinner, who leads the head-to-head 6-5, has also served exceptionally well. A change to his technique - shortening his ball toss and taking more time with his motion - has reaped rewards.

He has hit 97 aces to 16 double faults in his five matches so far - and against big-server Jan-Lennard Struff in the quarter-finals, won 84% of his first-serve points.

After a real scare in the first round, where he came back over five sets agianst Miomir Kecmanovic, Italy's Sinner has grown into the tournament.

Sinner also coped well in the heat against Struff, having struggled in the warm humid temperatures of Paris last month.

The Italian went out in the second round of the French Open in the extreme heat, having admitted he "got lucky" when the heat rule was enforced at the Australian Open when temperatures hit 40C in his third-round match.

He has cut a more relaxed figure as the tournament has gone on - and, like Djokovic, will have benefited from two days rest after their 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." (BBC)