It was Thiem, who made his debut at the event past year, who started nervously, allowing Dimitrov to ease to the opening set. This year the crew includes both Roger Federer and the highly regarded Roger Federer cover band that performs under the name Grigor Dimitrov.
He next plays Nadal's first match conqueror, Belgian David Goffin, on Wednesday before completing the pool stage against Spanish tournament reserve Pablo Carrena Busta on Friday.
Bulgaria's Grigor Dimitrov beat fourth-seeded Dominic Thiem of Austria at the ATP World Tour Finals 6-3, 5-7, 7-5 at the O2 Arena here on Monday.
Dimitrov passed up his first chance to serve out the match in a tense third set but made no mistake two games later to triumph in the day's opening match in London.
Having dominated a first set in which his opponent managed to get just 33 per cent of his first serves into play, Dimitrov should have won in straight sets.
"There's always a little bit of tension before every match".
"I'm not going to lie, I was pretty nervous in my first match out here", Dimitrov said.
Facing break point at 5-5 in the second, Thiem produced a forehand victor to deny Dimitrov an opportunity to serve for the match, and then broke to love in the following game to level.
Serving for the match at 5-4, however, he wobbled again as a framed backhand beyond the baseline allowed Thiem a reprieve.
In the pair's most recent meeting, in Madrid, Thiem won after saving five match points and it appeared another remarkable escape might be on the cards.
A shanked forehand wide gave Thiem a set-clinching break at 6-5 but Dimitrov again pulled ahead in the third, when a sensational drop-volley put him a break and 4-3 up.
"It was kind of a similar situation".