The 21-year-previous became the first Greek victor at the time-ending event, beating Thiem 6-7 (6-8) six-two 7-six (7-4) in a typical showdown at the O2 Arena on Sunday.
In an interview after beating Roger Federer in the semi-final, Tsitsipas said: "I remember myself being one of these kids here, watching the event, and for me, it was just, I could never picture myself standing here", said Tsitsipas. He has charmed the group at London's O2 Area all week and earned his place within the closing with a powerful straight-sets win in opposition to six-time champion Roger Federer.
The powerful one-handed backhands were out in force from both players in the tie-break, but it was a frightful, wafted forehand from Thiem which handed Tsitsipas the initiative, and a memorable victory was wrapped up in two hours and 34 minutes.
Just a year after winning the NextGen Finals, for players of 21 and under, Tsitsipas mixed it with the very best on the bank of the Thames and came out on top.
"I have no clue how I performed so well in the next established", said Tsitsipas, the youngest victor of the tournament because Lleyton Hewitt in 2001.
Defeat is a bitter disappointment for Thiem, who won five titles this year, including the Indian Wells Masters, and reached a second successive French Open final.
Tsitsipas was better than me.
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A sprightly Tsitsipas put that behind him by taking a firm grip of the second set, putting away a brilliant forehand victor after toying with Thiem to go a double break up at 3-0. Tsitsipas held easily after which produced a forehand down the road to interrupt once more, with Thiem threatening to unravel.
Thiem, who like Tsitsipas plays with a single-handed backhand, stemmed the tide but could make little impression on his opponent´s serve and lost the set 6-2. Thiem was rocking once more at first of the deciding set.
Austria's Thiem grabbed the early advantage after a slugfest of an opening set that saw the players go virtually shot-for-shot. He saved two break factors within the first recreation however appeared more and more shaky and was damaged to path 1-2.
The Greek was now effectively on prime however out of nowhere Thiem was degree at Three-Three after Tsitsipas put a backhand broad. Then he doubled up to put himself in a strong position to close out the set which he duly did, as he ran out the second set victor, 6-2. But Thiem battled back to 4-4 only to hit a routine forehand long and hand the Greek a chance to serve out the match. Tsitsipas has competitors amongst his friends as the brand new technology try and dislodge Federer, Novak Djokovic and Rafael Nadal, who've dominated the boys's recreation for the previous 15 years.
It is the fourth year in a row that there is a first-time champion at the ATP Finals, following wins by Andy Murray in 2016, Gregor Dimitrov in 2017 and Alexander Zverev last year.
But Tsitsipas, who has beaten every member of the "Big Three" in 2019, will go into the new season with the belief he can take the next step towards the apex of the men´s game.