In the second-set tie-break, Thiem blew four match points - on his first match point he served a double fault.
While there will be obvious disappointment for Thiem in losing in the final for a second consecutive year, the 27-year-old has improved his all-round game in another solid year.
'I am not relieved, it's just disappointing to finish the season with a loss like this, ' said the 33 year-old Serbian. "But I guess it wasn't developed because we just did not have the cases like this previously".
The 27 year-old Austrian finally came through a unusual encounter with the world number at the O2 Arena today, beating him 7-5 6-7 7-6 in almost three hours and on a sixth match point.
(MENAFN - NewsBytes) World number four Daniil Medvedev trounced veteran Rafael Nadal 3-6, 7-6 (7-4), 6-3 to reach the grand finale of ATP Finals.
"I think it's super for tennis".
It seemed like he had exhausted the wily Russian's box of tricks when he served for the match at 5-4 in the second set but Medvedev, 10 years his junior at 24, stormed back to win 3-6 7-6 (4) 6-3. In the last 70-minutes of play, Medvedev turned the match on its head, running away with the final four games to score a 3-6, 7-6 (7-4), 6-3 win. It was so much on the edge that match, like every single match here.
Azerbaijan reclaims disputed land in Karabakh peace deal
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Nadal held and then broke Medvedev with some incredible passing shots and that world-famous determination of his which has made him the joint-most successful men's player in history. This evening he will seek to become the sixth different victor of this event in six years, a sequence going back to 2015, when Djokovic took the last of four straight titles here. We are starting to make our mark.
"It was a mental battle, I got so tight in the second-set tiebreak", Thiem, who avenged his defeat by Djokovic in this year's Australian Open final, said on court.
"Playing for the final here at the Nitto ATP Finals is also something very special and I thought that after my first big title in NY, maybe I'm going to be a little bit more calm, but that was a mistake, I guess".
The third game was also pushed to a tiebreaker, which was started by Thiem but he went on to lose the next three points. "Well done for him and not enough good for me". I lost a big opportunity. He is not the most ruthless closer of matches when front running, but hits the ball off the ground with such power that he is capable of ascending all the way to the top in 2021.
"I sincerely wish for him that he overcomes this soon and that he can focus on his life and tennis career".
Medvedev, who is 1-3 against Thiem, has that momentum going again.
Thiem stumbled and tumbled to the court in the next game, but appeared to be OK. "I'm feeling confident. I think Rafa is also".