Coming into the game, all the money was flowing Boston's way, as they were riding their 16-game winning streak, including a win over the defending champs the Golden State Warriors.
But, as with any streak, fatigue does start to take its toll, and heading to Miami to play couldn't have helped, as the Miami Heat have broken Boston's winning streak with a 101-99 victory over the East's number one seed.
Both Goran Dragic and Dion Waiters came out of the gates firing, combining for 54 points as Miami's backcourt had their way with Jaylen Brown and Kyrie Irving.
The Heat had a scare late in the fourth quarter, as the Celtics rallied together behind rookie Jayson Tatum's 18 points and 7 rebounds, as well as this incredible block to shift the momentum.
Miami was up by 18 points at one stage during the game, but that lead was almost cut to nothing late in the fourth.
Other than Tatum, Irving was his usual self, scoring 23 points, and was helped by both Jaylen Brown and Marcus Morris, who both chipped in 14 a piece, but alas, it wasn't enough to continue the streak.
With this huge win, Miami move up to 8-9 on the season, whereas Boston fall to 16-3, still leading the Eastern Conference standings.
Boston will be facing the Orlando Magic next in hopes of getting back to their winning ways.