Djokovic fell behind in both tiebreaks but, having saved a set point at 5-6 when Raonic botched an attempted forehand victor, a brilliant backhand half-volley victor from the reigning Australian and French Open champion late in the tiebreak was one of the best and most important of the night.
Novak won just two points more in the end and Milos certainly can lament on the fact he had 7 break points, nicely ordered throughout the sets, but just like always against Novak he failed to deal with the mental pressure and prevail in that segment of the game.
The first stayed neck-and-neck as Raonic's serve looked untouchable; the second saw both players breaking serve twice, a frazzled Djokovic tensing up and ceding his early lead, and a confident Raonic exploiting that with some timely shot-making.
Questioned about his temper Sunday after smacking a ball in the direction of his coaches and not far from the crowd in his win against Thiem, Djokovic was mostly composed despite his inconsistent display - although he did punish both feet with his racket when dropping serve at 2-1 in the second set. The "Djoker" needed all of his famed elasticity and ability to turn defence into attack throughout a superb first set in which Raonic unloaded a series of forehand winners and crunching serves. "But, you know, I think I should have done better there".
In the tenth game, where Monfils sent down his fifth double fault of the match, Thiem struck on his first breakpoint opportunity which was also match point, Monfils' backhand floated long at the end of the 38-minute decider.
Winning the first set gave me some wings and I was twice a break up in the second.More news: How the Dow Jones industrial average fared on Thursday
The 23-year-old Austrian used one break at 3-2 to cruise through the opening set before the match turned into something strikingly familiar.
"I was fortunate to get through the first set tie-break".
After a second defeat in two matches, it is fair to wonder whether Monfils will play his final round robin match Thursday against Djokovic, with alternate David Goffin waiting in the wings. I have a chance in two days, 36 hours, 48 hours to come back and do better than I did today.
LONDON-Canadian Milos Raonic, touch and go to play in the ATP finals because of a leg injury, was a victor in Sunday's opening match. "I'm very happy with the win", he added. "Potentially I have the chance to reach the semifinals". Third set was very tight, and he helped me a little bit in the last game of the match.
Monfils also lost to Canada's Milos Raonic in his opening match on Sunday and was hobbling at times, putting his continued participation in some doubt.