Defending champion Stefanos Tsitsipas will battle it out with Rafael Nadal for a place in the semi-finals of the Nitto ATP Finals after saving a match point to beat Andrey Rublev.

Tsitsipas' 6-1 4-6 7-6 (6) win means Dominic Thiem qualifies for the last four after his brilliant victory against Nadal earlier, with the Greek and the Spaniard heading to a winner-takes-all clash on Thursday.

Tsitsipas was dejected after losing his opening match to Thiem but reacted in just the right manner here, wrapping up the first set after only 19 minutes.

However, Rublev, making his debut in the tournament, produced a spirited response and had a match point in the deciding tie-break only to double fault.

Tsitsipas said in his on-court interview: "It was an unbelievable match from both sides. We produced some incredible tennis. The relief at the very end of putting all that effort and all that fight on the court paid off.

"I'm very happy that I showed determination and a willingness to not give up when he had match point. I produced some really good tennis and that helped me take the win."

Russian Rublev qualified for the tournament for the first time by virtue of winning more titles and matches than anyone else this season.

The 23-year-old settled into the match at the start of the second set but Tsitsipas appeared untroubled until he came to serve to stay in the set at 4-5 and was broken to love.

The Greek tried to wrestle back the ascendancy early in the decider but was unable to take any of five break points in the third game and Rublev grew in confidence as the contest headed to a deciding tie-break.

Tsitsipas looked in control at 5-2 but lost the next four points, with Rublev putting together a tremendous rally to force a match point.

He let Tsitsipas off the hook with just a second double fault of the match and his hopes of progressing are now over.