The Austrian only qualified for the event in London as a result of Rafael Nadal's withdrawal in the autumn, but he has given himself a chance to reach the semi-finals with a 6-3 1-6 6-4 over Monfils.
One break of serve was enough for Thiem to take the opener in just 26 minutes and with Monfils carrying a leg injury, it seemed that the debutant would run away with the match.
However, Monfils blitzed through the second set by recording three breaks of serve, and he had the momentum heading into a crunch decider with both men realistically requiring victories to remain in the tournament.
The opening nine games went with serve but in the 10th, Monfils registered three double faults on the way to conceding the match to Thiem.
Thiem will now face Milos Raonic in his final match on Thursday. body check tags ::