Novak Djokovic made a winning start at the ATP Finals as he chases a record-equalling sixth title and overhaul Rafael Nadal as world number one.
The Swiss star will take on Matteo Berrettini on Tuesday and Novak Djokovic on Thursday.
The Swiss 20-time Grand Slam victor recovered quickly from being broken in the first game of the match but the Austrian fifth seed repeated the dose in the 11th game and took the opening set 7-5.
But all eyes are on a mouth-watering repeat of the Wimbledon final in Group Bjorn Borg later in the week - the top two players from the four-man group qualify for the semi-finals.
Thiem had beaten the 20-time grand slam champion twice this season and completed the hat-trick with a 7-5 7-5 victory at the O2 Arena on Sunday.
Berrettini's forehand was responsible for 18 of those errors, including two shots into the net to hand Djokovic the only two breaks of the first set.
Thiem was extremely impressive during his drubbing of Federer and showed both sides of his game to defend a crucial break point early in the second set and clinically finish the showdown in the tense final stages.
Djokovic, who won the Australian Open and Wimbledon this season, must reach the final in London to have any chance of overtaking Nadal at the top of the rankings.More news: E-cigarettes don't increase teen smoking chances, study says
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The second seed saw off the challenge of Berrettini in just 62 minutes, converting five of seven break points and out-serving his 23-year-old opponent.
"It was a great performance and special to get my first win in an opening group match in the Finals", Thiem said.
"It's not easy for Matteo, playing his first tour finals match, I knew he'd be a bit more nervous at the beginning, I tried to use my experience".
Earlier in the day Djokovic thrashed Matteo Berrettini 6-2, 6-1 so he will take on Thiem on Tuesday.
Germany's Alexander Zverev shocked Djokovic in last year's final at the O2 Arena but the Serbian is the firm favourite to equal Roger Federer's tally of six ATP Finals titles after cantering to victory at last week's Paris Masters.
Nadal, who has never won the event, is bidding to clinch the year-end number one spot in the ATP rankings for the fifth time.
Nadal, Daniil Medvedev, Stefanos Tsitsipas and Zverev are in Group Andre Agassi.