Three years later, Thiem is facing that same player in the final of the season-ending tournament.
Thiem beat defending champion Alexander Zverev in the semifinals on Saturday after Stefanos Tsitsipas ousted six-time winner Roger Federer, setting up a title match between two players who first met each other at the O2 Arena under very different circumstances.
"I just saw a picture before that I practiced with him the first time I played here 2016," Thiem said after beating Zverev 7-5, 6-3. "He was a hitting partner here. It's an amazing story for both of us. … We didn't think that only three years later we were going to face each other in the final."
The now 21-year-old Tsitsipas had the most eye-catching win on Saturday, saving 11 of 12 break points to beat Federer 6-3, 6-4. He also took advantage of an error-filled performance from Federer, who continually put his opponent under pressure only to come up short when it mattered.
"I'm proud of myself, how hard I fought today, how concentrated I stayed in the breakpoints," said Tsitsipas, who reached the biggest final of his career. "Didn't crack under pressure. I was very composed and very mature in my decisions."
In the evening match, Zverev doubled-faulted on set point to hand Thiem a 1-0 lead and was then broken for the second time to make it 4-2 in the second set.
Thiem saved two break points in the next game to hold for 5-2, and then served out the match on his first attempt, clinching the victory with a forehand winner on match point.
The final pairing also ensures that the tournament will have a first-time champion for the fourth year in a row.
Federer and Novak Djokovic combined to win the ATP Finals nine times in 10 years between 2006-15 before Andy Murray broke that streak and Gregor Dimitrov won it in 2017.
Zverev was trying to repeat last year's title win but couldn't convert any of the four break points he forced against Thiem. As he sat down for the changeover after his double-fault to end the first set, he slammed his racket down so hard it bounced along the court and came to rest behind the baseline.
He was also frustrated with the way he gave away a cheap break in the second set.
"I broke myself. I missed two overheads one meter away from the net," Zverev said. "It was just a bad game. Yeah, the match was over a few games later, so what can I do?"
Thiem beat both Federer and Djokovic in the group stage just to reach the semifinals for the first time in four attempts at the tournament.
Tsitsipas, who is making his first appearance at the event, saved all six break points he faced against Federer in the first set. That included two at 5-3, when he needed seven set points before finally winning a marathon game.
He broke again for a 2-1 lead in the second, then saved three break points from 0-40 in the next game before Federer finally converted his fourth to level the set.
But Tsitsipas broke again straight away with a forehand winner and then saved two more break points from 15-40 down when serving for the match at 5-4.
He didn't give the 38-year-old Federer any more chances of a comeback, serving out the match with an ace.
"No doubt I had my chances," Federer said. "I'm just frustrated I couldn't play better. And when I did and fought my way back, I threw it away again."
The 17-year age gap between the two players was the largest in the history of the tournament.
For Federer, it was a surprisingly erratic performance after he played near-flawless tennis to beat Djokovic in straight sets on Thursday to reach the semifinals.
He finished that match with five unforced errors - including two double-faults - but had 26 in this match.
Federer was especially unhappy with the service break in the first set, when he also missed two fairly routine overheads to gift his opponent the early lead.
"Getting broken with missing two smashes in one game, that hasn't happened in a long, long time. Or ever," Federer said. "So that was tough."
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