The sixth seed got off to the ideal start, winning his opening round-robin match in the Pete Sampras Group as he was able to keep his nerves in check to beat the Austrian 6-3 5-7 7-5.
Neither player was in top form, with serving an area in obvious need of improvement, but Dimitrov always looked the more likely as the Austrian's sorry run - he now has just two wins in his last seven tour matches - continued.
Instead of sending the match in the tie break, Dominic played an bad service game to get broken again and fell 6-5 behind, hitting costly double faults to reduce his chances, and Dimitrov sealed the deal with a solid hold in game 12. The Bulgarian would outlast the world No. 4, 6-3, 5-7, 7-5.
Nadal's appearance has been in doubt after a knee injury forced the 31-year-old Spaniard to withdraw midway through the Paris Masters tournament.
However, the third set proved a step too far for the 16-time Grand Slam champion, who began to struggle with his movement and was in visible pain as Goffin finally claimed victory.
It was Thiem, who made his debut at the event previous year, who started nervously, allowing Dimitrov to ease to the opening set.
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In a topsy-turvy decider, Dimitrov served for the match but blew his chance as Thiem broke back to level the set at 5-5.
"Every year you learn more about yourself, about the game, about the players", Dimitrov said. "Also because it's your first match, it's against a really, really good opponent, so you're very tense".
Again Dimitrov wobbled, squandering two match points at 40-15 with a double-fault and a tight forehand error, but he finally bagged the win when Thiem looped a backhand long.
But rather than Dimitrov, Thiem it was who wilted. I was like, Not again, definitely not again.
"I think the best is yet to come".