Instead of world No.1 Novak Djokovic facing second-ranked Rafael Nadal for the season-ending ATP Finals title it will be world No.3 Dominic Thiem against fourth-ranked Daniil Medvedev.

Nadal had won 71 matches in a row when grabbing the opening set and he served for the victory in Saturday’s semi-finals when leading 5-4 in the second set.

However, Medvedev broke at love there and came all the way back to win 3-6 7-6 (7-4) 6-3.

That came after Thiem frittered away four match points in his clash with Djokovic because he was “tight and nervous” during a second-set tiebreaker.

Thiem gathered himself and eventually reeled off seven of the match’s last eight points, after trailing 4-0 in the third-set tiebreaker, to prevail 7-5 6-7 (10-12) 7-6 (7-5) in an instant classic.

“What he did from 0-4 in the third-set tiebreaker was just unreal,” Djokovic said.

“I don’t think I played bad. … He just crushed the ball and everything went in.”

Thiem ended Djokovic’s bid for a record-tying sixth ATP Finals trophy, while Medvedev prevented Nadal from continuing to pursue the most significant title the 20-time major champ hasn’t won.

A year ago, Medvedev went 0-3 in round-robin play at the ATP Finals, while Thiem was the runner-up to Stefanos Tsitsipas.

On Sunday, the 24-year-old Medvedev and 27-year-old Thiem, who won the US Open in September for his maiden career grand slam title, aim to become the ATP Finals’ sixth different champion over the past six years.

Djokovic, already assured of finishing the year ranked No.1 for a record-tying sixth time, was trying to match Roger Federer’s mark of six ATP Finals successes.

In the second set against Djokovic, Thiem’s ace put him ahead 6-5 in the tiebreaker, one point from victory.

But Djokovic saved that initial chance with a service winner.

Thiem’s next opportunity to win came at 7-6 but he double-faulted.

“I was, like, so tight in my whole body,” Thiem said.

The third was at 9-8, when he pushed a down-the-line forehand wide. At 10-9, Djokovic erased No.4 with a forehand that landed right on a line.

That began a three-point run by Djokovic to steal the set.

But Thiem regrouped and his 300th career tour-level victory made him only the second man with at least five wins each against the Big Three of Djokovic (5-7 career mark), Federer (5-2) and Nadal (6-9). Andy Murray is the other.

“If you beat these guys it gives you a huge boost of confidence,” Thiem said.