Dominic Thiem claimed his maiden victory at the ATP World Tour Finals with a 6-3 1-6 6-4 victory in London on Tuesday.
The Austrian eighth seed is competing at his first Finals but capitalised on an apparent fitness issue suffered by his opponent to clinch the opening set.
Monfils rallied to claim a comfortable victory in the second, but three double faults at 5-4 down in the decider ultimately cost him.
Thiem was quickest to find his stride and claimed the first set with a single break of serve.
Monfils looked to have picked up an injury late in the opener and was moving gingerly as Thiem closed out the set to love.
However, the Frenchman shook off the problem to race into a 4-0 lead in the second set, halted only when Thiem converted a third break point to pull back to 4-1.
Monfils soon got back on the front foot to level the match at one set all before both men pulled out all the stops for the crucial third, exchanging a series of wonderful strokes as they matched each other blow for blow.
Monfils battled hard for a vital hold to preserve parity at 4-4, but he eventually sent a backhand long in a poor final game to hand the match to Thiem on a plate.
The win follows Thiem's opening defeat to Novak Djokovic, while Monfils remains bottom of Group Ivan Lendl with two defeats from as many matches.
Thiem  bt Monfils  6-3 1-6 6-4
Monfils - 30/23
Thiem - 20/19
Monfils - 8
Thiem - 3
BREAK POINTS WON
Monfils - 3/3
Thiem - 3/8
FIRST SERVE PERCENTAGE
Monfils - 49
Thiem - 66
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Monfils - 82/46
Thiem - 72/50
Monfils - 74
Thiem - 74