Novak Djokovic says he has not yet decided whether he will compete at January's Australian Open.

Organisers yesterday confirmed that all players must be vaccinated against COVID-19 if they want to compete at the season's opening grand slam event.

"We'll see. We'll have to wait and see," world number one Djokovic told reporters after his semi-final defeat by Alexander Zverev at the ATP Finals on Saturday.

"I haven’t been talking to them, to be honest,” the top-ranked Djokovic added.

"I was just waiting to hear what the news is going to be and now that I know we’ll just have to wait and see."

Djokovic, who has declined to say publicly if he has been vaccinated, is currently tied with Roger Federer and Rafa Nadal with 20 grand slam titles.

The Australian Open, which he has won nine times, is seen as a golden opportunity for him to take the lead over his rivals.

Nadal confirmed he would play at Melbourne Park in January but Federer, who like the Spaniard is also fully vaccinated, will miss the tournament as he recovers from another knee surgery. 

Djokovic was beaten 7-6 (8/6), 4-6, 6-3 by Zverev, who set up a final showdown with Daniil Medvedev at the ATP Finals.

After splitting the first two sets, unforced errors began to creep into Djokovic's game in the third and he sent a backhand long to hand Zverev his first service break of the match for a 3-1 lead.

Two games later, the 24-year-old German appeared to get tight, taking speed off his first serves and pushing his groundstrokes, which opened the door for the resilient Djokovic to break back.

But Zverev pounded a second serve Djokovic could not handle to grab a 5-2 lead before sealing the match with his 14th ace.

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The victory prevented Djokovic from winning a sixth title at the season-ending tournament, which would have tied him with Roger Federer's record haul. Zverev won the title in 2018.

Medvedev crushes Ruud to reach final 

Earlier in the day, defending champion Medvedev stormed into the final after crushing Norway's Casper Ruud 6-4, 6-2.

Medvedev started strongly against Ruud, breaking early to take a 2-1 lead in the first set and dictating play from the back of the court as Ruud struggled to contend with his opponent's deep groundstrokes.

The Russian won 85 per cent of his first-serve points as he wrapped up the opener in 42 minutes.

The second set followed a similar pattern to the first, with Medvedev racing into a 4-2 lead before closing out the match in 80 minutes, sending down 17 winners and making just 14 unforced errors in the one-sided victory.

The 25-year-old US Open champion said one key to success was to never let up regardless of what the scoreboard says.

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"As soon as you are a set and a break up you feel like you are in control, but that is when the danger is," Medvedev said.

"You need to stay focused and fight for every point. In the last game I had 0-30 on my serve and, until the last point, it is never over. So I am happy I was able to finish it."

Medvedev cruised through the round-robin stage of the season-ending tournament with wins over Hubert Hurkacz, Zverev and Jannik Sinner before dispatching Ruud.

"I think all the matches were a great level. Different opponents and different styles," he said.

"I managed to win all the matches. It was not an easy match (today). When you are in the final you can't complain, so I am just looking forward to tomorrow and hopefully I can have my best match."

The result marked world number two Medvedev's ninth win in a row at the ATP Finals and he improved his head-to-head record over Ruud to 3-0.

World number two Medvedev holds a 6-5 lead in head-to-head matches against Zverev.

Reuters/AP