Terry Rozier finished with 23 points, Marcus Morris had 21, Jaylen Brown scored 20 and Jayson Tatum added 19 to become the first Celtics rookie since Brian Shaw in 1989 to score 15 or more in his playoff debut. Tatum also pulled down 10 rebounds.
"We've been down a lot (this season), a lot of injuries, and we've always found a way (to win)," Horford told TNT after the game.
Giannis Antetokounmpo had a game-high 35 points and 13 rebounds and Khris Middleton scored 31 for Milwaukee, which hasn't advanced past the first round in seven straight postseason appearances dating back to 2003.
Antetokounmpo and Middleton were the first pair of Bucks teammates to score 30 apiece in a playoff game since Bob Dandridge and Brian Winters did so in 1976.
Milwaukee and Boston split their four-game, regular-season series, with each team winning once on the other's home court. It is the first playoff meeting between the Bucks and Celtics since 1987, when Boston won a seven-game series in the East semifinals.
Game 2 of the series is Tuesday in Boston.
Tatum blocked Malcolm Brogdon's layup with 26.4 seconds left in overtime and the Bucks down 108-106. Brogdon collected the rebound and found Tony Snell, who missed a corner 3-pointer with 21.9 on the clock.
Terry Rozier hit two free throws with 18.8 left to make it a four-point game.
Antetokounmpo made one of two free throws at the 14.8-second mark, then committed his sixth and final foul fighting for a loose ball with Rozier, who made one of two free throws with 13 ticks to go.
Boston salted the win at the line after that.
Up 70-66 entering the fourth quarter, Boston took a double-digit lead at 86-76 on Morris' put-back dunk with 4:27 remaining.
Antetokounmpo hit two free throws with 1:22 to play to trim the Celtics' lead to 91-89 entering the final minute.
Brogdon hit a 3-pointer to tie the game at 96 with 10.3 seconds to go before Terry Rozier's go-ahead trey with 0.5 seconds on the clock.
Middleton hit a desperation 3-pointer from near mid-court as time expired to send it to overtime.
The Bucks led 47-44 at halftime.
Already without injured stars Kyrie Irving and Gordon Hayward, the Celtics also began their playoff push without defensive centerpiece Marcus Smart (right thumb surgery). However, Smart said he might be able to return for a potential Game 7 against Milwaukee.
--Field Level Media
Milwaukee | Boston | |
Giannis Antetokounmpo 35 | Scoring | Al Horford 24 |
Giannis Antetokounmpo 7 | Assists | Jaylen Brown 4 |
Giannis Antetokounmpo 13 | Rebounds | Al Horford 12 |
Giannis Antetokounmpo 13 | Free Throws Made | Al Horford 13 |
Giannis Antetokounmpo 3 | Steals | Jayson Tatum 3 |
John Henson 6 | Blocks | Al Horford 3 |
Team | Points | FG% | 3PM-3PA | FTM-FTA | Assists | Rebounds | Blocks | Steals | Turnovers |
Milwaukee | 107 | 48.2 | 8-21 | 19-24 | 22 | 42 | 8 | 8 | 20 |
Boston | 113 | 41.5 | 11-26 | 24-27 | 20 | 45 | 5 | 8 | 15 |