A valiant second-half effort ended in sheer despair for the Boston Celtics on Sunday.

Trailing by as many as 20 points in the third quarter, the Celtics clawed back against the Milwaukee Bucks, but fell 104-102 in a matinee at BMO Harris Bradley Center in Game 4. The two sides’ first-round NBA playoff series now is tied heading back to Boston.

Jaylen Brown had a game-high 34 points. Jayson Tatum (21), Marcus Morris (13), Al Horford (13) and Terry Rozier (10) also were in double figures for Boston. Giannis Antetokounmpo was one of four Bucks with double-digit scoring, posting 27.

Here’s how it all went down:

PG: Terry Rozier
SG: Jaylen Brown
SF: Jayson Tatum
PF: Al Horford
C: Aron Baynes

In the first quarter of Game 3 the Celtics quickly put themselves in a hole, thus making it imperative they come out strong Sunday. They did that early, but things again fell by the wayside as they trailed 24-17 after one.

The Celtics were up 12-5 by the midway point of the quarter, but with 2:36 left the Bucks had reclaimed the lead. Things took a poor turn for Boston at the end, as Milwaukee finished the quarter on a 6-0 run.

Khris Middleton backed his way into the paint as the clock ticked down, turning and draining a six-foot bank shot to make it 22-17 with 1.4 seconds to play. Morris then tried to roll the ball to Shane Larkin to give Boston a chance at a buzzer-beater, but Matthew Dellavedova charged in and scooped it up, making a quick layup to beat the buzzer and electrify the home crowd.

Things didn’t get much better for the Celtics in the second quarter, as the Bucks took the momentum from their late first-quarter surge and used it to dominate. If not for some contributions from Brown, things could have been much worse than the 51-35 lead the Bucks went into the half with after outscoring the C’s 27-18 in the second.

With an 8-2 run over the first three minutes of the second, Milwaukee quickly opened their advantage to 13 points. Boston at times would cut into that lead, with Brown contributing 12 points — 10 of them in the last 5:33 — but the Bucks often had an answer to keep their lead in the neighborhood of 15.

Brown entered the half with a game-high 15 points, trailed by Middleton at 12. As a whole, however, the Celtics were horrid from the field, shooting just 30 percent.

Things got worse to start the third for the Celtics, but they then got much better as they cut Milwaukee’s advantage to 75-67 heading into the final frame.

With 7:37 left in the quarter, the Bucks extended their advantage to a game-high 20 points, but shortly after that point, the Celtics showed some life. The C’s used a 12-2 run to cut the lead to 10 with 4:33 left, and continued chipping away through the rest of the frame, ultimately outscoring the Bucks 32-24.

Tatum stepped up and dropped 13 points in the frame, while Brown added nine more of his own.

Boston came into the fourth hot, dropping a quick five points in the first 1:19 to cut the deficit to three. With 9:03 to play, Morris drove the lane and tied the game at 77.

After both sides went back-and-forth throughout the period, the Celtics claimed their first lead since the opening quarter on a Tatum jumper with 52 seconds left to put the Celtics up 100-99. With 33 seconds left, Malcolm Brogdon gave the Bucks the lead back with a corner 3-pointer.

Trailing by two, Horford was fouled and hit both of his free throws to tie the game at 102 with 29 seconds left. The Bucks answered with an Antetokounmpo tip-in with 5.1 seconds left.

On the other side of the timeout, Morris got the ball off the inbound, took — and missed — a would-be game-tying jumper. It was reviewed for a foul, but none was discovered and the game was over.

What proved to be the game-winner.

The series will return to TD Garden for Game 5 Tuesday. The tip-off time still is to be determined.

Thumbnail photo via Jeff Hanisch/USA TODAY Sports Images