Grigor Dimitrov edges Dominic Thiem in opener

The world number six briefly threw away a final set advantage – being broken at 5-4 when serving for the match – but he responded brilliantly to secure a 6-3 5-7 7-5 victory in Group Pete Sampras.

"The first time I got broken (at 5-4) I thought 'okay let's get back up and try again'," Dimitrov, who enjoyed some noisy support from a contingent of Bulgarian fans, said on court.

"I'm living my dream right now and I wanna keep on going."

The opening two sets saw a solitary break of serve apiece, but it was a turbulent affair from 3-3 onwards in the decider.

First, Dimitrov broke with a beautiful deft volley at the net, before fighting back from 0-30 in the next to consolidate his advantage.

But Thiem hit back, halting the Bulgarian as he served for the match – only to then immediately donate his serve with two untimely double faults.

And Dimitrov finally triumphed after two hours and 22 minutes, serving out victory via a nervy deuce.

World number one Rafael Nadal opens his quest for a first ATP Finals title later against Belgium's David Goffin.

Nadal's appearance has been in doubt after a knee injury forced him to withdraw midway through the Paris Masters tournament.