The Boston Celtics are on upset alert.

Giannis Antetokounmpo scored 27 points, including a game-winning tip-in with 5.1 seconds remaining, and Khris Middleton added 23 to lead the Milwaukee Bucks to a 104-102 win Sunday in Game 4 of the teams' first-round Eastern Conference playoff series. Each team has two wins as they head back to Boston.

When the Celtics last played on their home floor, they took a 2-0 series lead and were pushing their way to a sweep. The narrative had shifted from whether the depleted squad could handle the upstart Bucks to where the statue of Brad Stevens was going to be erected.

But the games in Milwaukee altered the series.

The Bucks led by 20 points with 7:37 remaining in the third quarter before the Celtics got back in the game with a torrid run. They erased the deficit over the next quarter-and-a-half and seized a 100-99 lead on Jayson Tatum's jumper with 52.4 seconds remaining.

Malcolm Brogdon hit a clutch three with 33.5 seconds remaining, and Al Horford tied the game at 102 with a pair of free throws on Boston's subsequent possession. The Celtics had one more chance to tie the game following Antetokounmpo's tip-in, but Marcus Morris missed a fadeaway jumper as time expired.

Jabari Parker and Thon Maker were again difference-makers off the bench, and their emergence has arguably been the biggest factor in Milwaukee's shifting the series. Parker, who combined for two points and six rebounds in Games 1 and 2, finished with 16 points and seven rebounds. He's been the Bucks' leading scorer among reserves in each of their victories.

Maker came up huge on the defensive end, blocking five shots for the second straight game to go along with eight points and two rebounds. He's the first player to have five blocks in consecutive playoff games since DeAndre Jordan in 2014, per Tommy Beer of Basketball Insiders.

Jaylen Brown spearheaded the Celtics' comeback with a game-high 34 points and eight rebounds. The second-year forward has made the most of the postseason spotlight, scoring at least 20 points in three of the first four games.

Tatum scored 18 of his 21 points in the second half, including 13 in the third quarter. He had scored just 18 points in Games 3 and 4 combined.

Horford (13 points, nine rebounds), Morris (13 points) and Terry Rozier (10 points, eight assists) were also in double figures for the Celtics. Brogdon finished with 10 points for the Bucks. Boston was held to 41.4 percent shooting, while Milwaukee made 52.1 percent of its shots.

The teams reconvene Tuesday night.