The duo was reunited for Game 5, and together, they helped put the Celtics on the verge of closing out their first-round series.
Horford had 22 points and 14 rebounds, Smart made an impact in his first game since mid-March and Boston beat the Bucks 92-87 on Tuesday night for a 3-2 lead.
Smart came off the bench and had nine points, five rebounds, four assists and three blocks in his first action since right thumb surgery. He played 25 minutes wearing a protective splint and had a key assist to Horford late from the bottom of a scrum beneath three Milwaukee players.
''I just tried to make sure I got him the ball and it was going to take everything I had,'' Smart said. ''They weren't taking it from me.''
Terry Rozier added 16 points and five assists for Boston. The home team has won all five games in the first-round series, and Game 6 is Thursday in Milwaukee.
The Celtics led by 16 before Milwaukee rallied to make it 74-70 with 7:33 left. Middleton scored later and was fouled to get Milwaukee within 84-79, but he failed to complete the three-point play.
A few possessions later, Smart managed to squeeze out the pass to Horford with three Bucks draped over him, and Horford easily dropped in a layup to make it 86-79 with 28.1 seconds left.
Milwaukee got within 87-84 after a layup by Eric Bledsoe, but the Celtics hit enough free throws the rest of the way to secure the win.
The Bucks entered the game shooting an NBA playoff-best 54 percent from the field, but they hit just 37 percent for the game (32 of 87) - a series low.
Boston needed an energy boost after slow starts in Games 3 and 4, and Smart gave the Celtics exactly that.
He checked in for the first time late in the first quarter. Less than a minute later, he deflected a pass and won a scramble on the floor for the loose ball.
A few possessions later, he was helping protect the rim, getting his hand in to deny an alley-oop dunk attempt by Antetokounmpo.
He was also active on the offensive end, helping drive the ball into the interior of Milwaukee's defense, like when he tossed an alley-oop to Horford late in the third quarter .
Boston made a change to a smaller lineup, shifting Horford to center in place of Aron Baynes and starting Semi Ojeleye at forward.
Ojeleye drew the primary responsibility of guarding Antetokounmpo and handled it well. Antetokounmpo had just 10 field goal attempts.
''It's on me. I had open shots, but they wasn't my shots,'' Antetokounmpo said. ''I think my teammates did a great job finding me, but coming the next game, Game 6, I've got to be more aggressive and make more plays.''
Bucks: F John Henson missed his third straight game with back soreness. ... Scored 15 points in first quarter. It was low for a quarter in the series, excluding the Game 1 overtime period (eight points).
Celtics: Finished with 18 turnovers. ... Outscored the Bucks 32-28 in the paint.
HE SAID IT
''It's win or go home, so I mean you can't leave nothing on the line. That's pretty much how you play this whole playoffs - you leave it all out there. Tonight we had too many lapses where we just didn't want it as bad as they did.'' - Middleton
DROUGHT AND A RUN
The Celtics went without a field goal from Rozier's 3-pointer with 9:14 left in the third until Jaylen Brown hit from beyond the arc with 4:26 to go in the period.
It allowed Milwaukee to go on a 12-2 run, trimming Boston's lead to 58-52.
Rozier and Eric Bledsoe got tangled up with 9:33 left in the third quarter. Rozier bumped Bledsoe near the baseline. Bledsoe responded with a hard shove. They then got face-to-face and had to be separated.
Bledsoe was assessed a flagrant foul and Rozier was issued a technical foul.
It was the continuation of some off-court chiding between the pair.
Rozier called Bledsoe ''Drew Bledsoe'' when asked about him after Game 1. Bledsoe has since responded by saying ''Who?'' whenever he's asked about Rozier.
''We're looking forward to Game (6),'' Bledsoe said. ''It's as simple as that.''
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