Dominic Thiem is through to his second consecutive season-ending ATP Finals decider after beating the world no.1 Novak Djokovic in a titanic last-four battle at the O2 Arena in London.

Thiem was trailing 4-0 in the final set tiebreak, but the reigning US Open champion battled back to take the semi-final against Djokovic; a five-time winner of this tournament, including four successive victories between 2012 and 2015.

In a battle lasting nearly three hours, Thiem squandered four match points in the second set tiebreak but eventually prevailed 7-5, 6-7 (10), 7-6 (5) to qualify for back-to-back finals at the ATP Finals. Thiem will face Daniil Medvedev in Sunday's final.

This result means that we are guaranteed a first-time winner of the event which sees the world's eight best men's players contest two round-robin groups of four where the top two from each group qualify for the weekend's semi-finals.

Thiem will be keen to avenge the circumstances of the 2019 final here where he lost the title to Stefanos Tsitsipas in a third-set tiebreak.

Meanwhile, Djokovic will finish the year as the world no.1 regardless of his exit on Saturday having secured this status by accumulating a sufficient number of points for the season at the recent Vienna Open.

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Thiem headed into this semi-final with a 7-4 losing record against Djokovic, the last defeat of which occurred at the Australian Open final in January which Djokovic won in five sets.

However, the Austrian beat the Serb in the group stage of last year's ATP Finals - also in a final-set tiebreak - and has previously beaten Djokovic at both the quarter-final and semi-final stages of Roland-Garros.

It has been a breakthrough year for Thiem. Having lost his first three grand slam finals, it was fourth-time lucky for the 27-year-old at the US Open final in September when he outlasted Alexander Zverev in a fifth set tiebreak to clinch his maiden major.

Watch as Colm Boohig, Adrian Barry and Eoin Sheahan preview the ATP Finals' decisive weekend and discuss the abuse allegations made against Alexander Zverev by his ex-partner, Olga Sharypova.

The triumph in New York has not curtailed Thiem's intensity and he has been in imperious form in London, exacting revenge against Tsitsipas in the tournament's opener while following that win up with a tight straight sets victory over Nadal.

Up until Saturday afternoon, that latter contest was a contender for the match of the tournament, but Djokovic had other ideas.

In a match of supreme quality, there was just one break point in the 51-minute first set which Thiem took at 5-6 to see the set out 7-5 courtesy of yet another blistering one-handed backhand winner down the line.

Thiem saved two consecutive set points at the tail-end of the second set, taking it into the first of two tiebreaks where the drama peaked.

Novak Djokovic in action against Dominic Thiem during their semi final match on day seven of the Nitto ATP Finals at The O2 Arena, London.

Djokovic saved a match point at 5-6 in the tiebreak. Thiem got another chance to win the match shortly after but double-faulted at 7-6. Then it was Djokovic's turn after a brilliant return of serve forced Thiem to hit the net in reply, but Djokovic failed to capitalise.

This led to a third match point for Thiem, but he could not see it out. He then missed a fourth chance to win the match in straight sets before Djokovic took his own fourth opportunity to take the match into a deciding set.

Another tiebreak followed in the final set and while it was less convoluted, there were equal levels of intrigue. Djokovic raced into a 4-0 lead and appeared destined for the final where he would look to win his sixth ATP Finals championship but his first in five years.

Alas, Thiem responded by winning five consecutive points to win the decisive tiebreak 7-5 and reach his second ATP Finals decider.

Dominic Thiem of Austria reacts during the semifinal match against Novak Djokovic of Serbia at the ATP World Finals tennis tournament at the O2 arena in London, Saturday, Nov. 21, 2020. (AP Photo/Frank Augstein)

After a 12-year run at the O2 Arena, the ATP Finals will move to the Pala Alpitour Stadium in Turin, Italy from 2021 for the following five years.

Prize money for the winner is down this year from circa $2.6m to $1.5m due to the loss of earnings in the absence of any crowd.

Despite this scenario and the truncated nature of the ATP Tour in 2020 due to COVID-19, the level of tennis this week in London has been befitting of the event's prestige in its final year at the O2.

Of the eight players competing, seven are unquestionably the world's present best with the only debatable exception being the Roland-Garros semi-finalist, Diego Schwartzman, who qualified in the absence of Roger Federer. The Swiss icon last played at the Australian Open having undergone knee surgery in June during the sport's global pandemic-induced hiatus.

Federer, 39, is aiming to return to the tour at the beginning of 2021.

He is now tied on 20 grand slam titles with Nadal after the Spaniard's record-extending 13th win at the recent French Open. Federer has not won a grand slam since the 2018 Australian Open before losing two championship points in last year's Wimbledon final to Djokovic, who trails the pair on 17 grand slams.