Novak Djokovic produced yet another scarcely-believable comeback to stun Carlos Alcaraz in a thriller and reach the Australian Open semi-finals.

The 37-year-old Serb defied injury, age and ranking to win 4-6 6-4 6-3 6-4 against his 21-year-old opponent in Melbourne.

"It was possibly one of the best matches of the tournament on the men's side," said Djokovic.

"It felt like a final of a Slam and I wish it was. We both gave it our all."

Djokovic, going for a record-extending 11th men's singles title, limped around Rod Laver Arena at the end of the first set with a problem in his upper left leg.

But, after going off court for treatment, he recovered impressively to continue his bid for a 25th Grand Slam triumph - which would be an all-time record.

Djokovic played aggressively until the pain wore off, which then allowed him to move more freely and take control.

Spanish third seed Alcaraz became increasingly animated as he struggled to figure out how to turn an engrossing contest back in his favour.

When seventh seed Djokovic sealed victory - at 00:57 local time after a battle lasting three hours and 38 minutes - he roared towards coach Andy Murray.

Djokovic and Alcaraz shared a warm embrace at the net after a highly-anticipated match which lived up to the expectation.

"When Novak plays at this level, it's really difficult. It was a really close match," said Alcaraz.

The defeat ends Alcaraz's hopes - for now, at least - of becoming the youngest man to complete the career Grand Slam.

Instead it is Djokovic who advances to the semi-finals, where he will face German second seed Alexander Zverev on Friday.

Djokovic produces a miracle - again

For so many years, Djokovic has defied the ageing process with his endurance and dexterity.

This was the latest example – at a time when few had predicted it.

Time, it seemed, had eventually started to catch up with a player who is aiming to become the oldest Grand Slam men's singles champion in history.

Last year was the first since 2017 – and only the second since 2010 – that he did not win one of the sport's four major prizes.

Doubts had already been raised about his ability to outlast the very best of his younger opponents – namely Alcaraz and world number one Jannik Sinner - over five sets.

His aura of invincibility at Melbourne Park was damaged last year when Sinner dominated their semi-final to end his 33-match winning streak at the venue.

Then Alcaraz walloped him in the Wimbledon final – albeit in a match that came less than six weeks after Djokovic had surgery on a tear in his knee.

However, Djokovic won his last encounter with Alcaraz, putting in a ferocious display at the Olympics to claim a gold medal which had previously eluded him.

A more patient approach in the first set on Tuesday was quickly replaced after he tweaked something lunging for a drop-shot.

Employing more aggression was a risk-reward strategy but necessary given his physical condition. It paid off.

Djokovic cracked 11 winners, including a rasping backhand on set point to level the match, as he successfully negotiated his way through a barrage of Alcaraz drop-shots.

Alcaraz's service games at the start of the third set continued to come under heavy fire. He buckled when Djokovic broke for 4-2, with the third seed losing serve again for 5-3 after he had seemingly rescued the situation.

Djokovic used the momentum to break in the first game of the fourth set - which ultimately proved decisive.

After surviving three break points and recovering from a host of long, energy-sapping points, Djokovic held his nerve to serve out a memorable win.

On being ready to play again on Friday, he said: "I'm concerned physically but if I can somehow manage to be physically ready, I'm mentally and emotionally as ready as I can be."

Alcaraz loses his way after 'controlling the match'

While many thought Djokovic was the underdog going into Tuesday's match, Alcaraz knew the size of the task he faced.

The four-time major champion pointed to Djokovic's greater experience and his "unbelievable" fitness, but insisted he would not be daunted by facing the man widely regarded as the greatest player of all time.

After a nervy start, he warmed to his task.

The intensity of Alcaraz's game, returning from seemingly-impossible positions with venomous power, seemed to take a toll on Djokovic.

But the younger player became befuddled as Djokovic's aggressive tactics, which particularly targeted Alcaraz's backhand, paid dividends.

"I felt like I was controlling the match and I let him get into it again," said Alcaraz, who has not gone past the quarter-finals in four Melbourne appearances.

"I'm going to say that was the biggest mistake that I made.

"In the second set I had to play a little bit better just to push him even more to the limit."