Neither have won the event before, so a first-time winner of the end-of-season finale is guaranteed.

Russian Medvedev has been in imperious form in London, and bounced back from dropping his first set of the tournament to record a come-from-behind win.

Spaniard Nadal - who despite 20 Grand Slam singles titles has never won the ATP Finals - was serving for the match at one point in the second set, but Medvedev broke and went on to win 3-6, 7-6 (7-4), 6-3. 

Thiem also needed three sets to reach the final, and his clash with Serbian world number one Djokovic lasted 2 hours 54min.

The Austrian won 7-5, 6-7 (8-10), 7-6 (7-5), making up for squandering four match points in the second-set tiebreak.

Thiem was the runner-up at this event last season, but 2020 has already been a year of firsts for the 27-year-old who won the US Open.

En route to his first Grand Slam title, Thiem beat Medvedev in the quarter-finals.

Thiem leads the pair's head-to-head record 3-1.

The final is scheduled for tomorrow, following the doubles final.

Austrian-French duo Jürgen Melzer and Édouard Roger-Vasselin are up against Croatian Nikola Mektić and Dutchman Wesley Koolhof in the doubles final.