The Philadelphia 76ers still have a steep hill to climb after their injury-riddled, dysfunction-filled, downright disastrous start to the season.

But Philly looked Wednesday like the team it hoped to be after its all-in offseason: a legitimate Eastern Conference challenger to the defending champion Celtics.

Boston, meanwhile, is now feeling the sting of its first two-game losing streak since April.

The Celtics lost to the Sixers 118-114 on Christmas Day at TD Garden after a late-game rally came up short. It was the second consecutive loss and fourth in seven games for Joe Mazzulla’s squad, which fell to 22-8.

“Listen, we’re playing inconsistent basketball,” Mazulla said. “We’ve got to be better at both ends of the floor. Got to be more consistent at both ends of the floor.”

Jayson Tatum led Boston with 32 points on 11-of-20 shooting and 15 rebounds after missing Monday’s loss in Orlando with an illness. Jaylen Brown finished with 23 points after an ugly first half, and Derrick White (17 points) and Al Horford (21 points) went a combined 10-for-20 from 3-point range for the Celtics.

Kristaps Porzingis played 13 minutes before leaving the game with an injury — the third he’s suffered since returning from offseason leg surgery on Nov. 25. The Celtics called it “left ankle soreness,” and Porzingis did not emerge from the locker room after halftime. The severity of the injury was unclear.

The 76ers got an excellent all-around performance from point guard Tyrese Maxey (33 points, 12 assists, three steals, one block) and superb 3-point shooting from Joel Embiid (4-for-5) and Caleb Martin (7-for-9), who both well outpaced their season-long averages. After starting the season 3-14, the Sixers have won eight of their last 11 to inch back toward contention.

Horford, stepping into the starting lineup in place of an injured Jrue Holiday (shoulder), hit two threes in the opening two minutes, including one as the shot clock expired. Porzingis added two of his own, starting strong from beyond the arc after an 0-for-4 showing against Orlando.

The two big men provided all of Boston’s scoring in the front half of the first quarter. No other Celtic got on the board until White sank a corner three more than seven minutes in, and Horford, Porzingis and backup center Luke Kornet teamed up to score 17 of the team’s 25 first-quarter points.

It was a disjointed quarter for Boston that featured some of the same ball-security issues that plagued them against the Magic. Tatum and Brown combined for nearly as many turnovers (four) as points (five) in the frame. The Celtics committed five as a team to Philly’s one and had a hard time corralling Maxey, who had 12 points in the first.

Philadelphia led 30-25 at the end of one, then more than tripled its lead during the second quarter, building a 16-point cushion that was the largest by any TD Garden visitor this season. Guerschon Yabusele — a 2016 Celtics draft pick who played his way back into the NBA by excelling for France at the 2024 Summer Olympics — scored eight straight points for the Sixers, who also got eight points in the second from Martin, a Celtics nemesis during his time with the Miami Heat.

Yabusele’s 12 points in the game matched the output of Boston’s entire bench, with top reserves Payton Pritchard and Sam Hauser going a combined 1-for-10 from the floor and 0-for-9 from three.

Tatum began to assert himself late in the second, making back-to-back 3-pointers as part of a 16-point first half. He also made a savvy play to throw the ball off a Sixers player while caught in a trap a halfcourt, setting up a Pritchard three later in the same possession. The Celtics cut their deficit from 16 points down to six before halftime.

Embiid, who had an injury scare pregame when he tumbled out of bounds during warmups, was unusually effective as a perimeter shooter in the first half. The embattled Sixers center came in averaging less than one made three per game, but he went 3-for-4 from deep before halftime. He and Maxey each scored 18 first-half points for Philadelphia.

Brown was dreadful over the first two quarters (1-for-8, two points, four turnovers) but locked in after half. He scored on two of Boston’s first three possessions of the third quarter, and a subsequent three by Tatum cut the Sixers’ lead to one, 66-65.

Four minutes later, Brown buried a three to tie the game at 74-74. Another by Tatum with 4:03 remaining in the third gave the Celtics their first lead since early in the first quarter. Brown poured in 14 points in the frame, with he and Tatum accounting for all but one of Boston’s made field goals.

Tied at 82 entering the fourth, the Sixers again threatened to overwhelm the Celtics by ripping off a 21-6 run that featured Martin’s fourth, fifth and sixth made threes of the evening. Philadelphia led by 15 with just over five minutes remaining.

But the Celtics weren’t done. After White and Martin traded 3-pointers, Boston scored 11 straight points. Tatum, White and Horford all scored during that surge, and the Celtics forced turnovers on three consecutive 76ers possessions — a pair of Brown steals and a backcourt violation on Maxey.

The two steals were the only ones Boston recorded in the game. The second led to a Horford dunk that cut the Sixers’ lead to 108-105 with 2:27 to play. The Celtics outscored the 76ers by 22 points with Horford on the floor, the best mark of any player on either team.

Boston couldn’t close the gap, however.

Back-to-back misses by Horford and Pritchard allowed Philly to rebuild a seven-point lead. The Celtics got that back to three with a steal and two buckets by Tatum, a Horford block and a Brown layup, then made it a two-point game after Brown followed up a pair of Paul George free throws with a 3-pointer with 4.2 seconds remaining. Boston immediately fouled Embiid, who looked clearly hobbled at times in the second half, and he made both to ice the game.