ORLANDO, Fla. — Boston’s Jayson Tatum opened the Utah Summer League and his pro career with a go-ahead basket to beat No. 1 pick Markelle Fultz and Philadelphia.

The game in Salt Lake City was the most anticipated contest thus far, as summer play heated up Monday with the Orlando Pro Summer League entering its third day and the Utah Summer League tipping getting started. The 24-team NBA Summer League in Las Vegas starts on Friday.

New York, Oklahoma City, Orlando, Dallas, Indiana and Miami were in action in Orlando on Monday. Boston, Philadelphia, Utah and San Antonio got going in Salt Lake City later on Monday.

A look at some of the notable action:

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DAZZLING DEBUT

Tatum’s jumper and the Celtics’ ensuing stop of Fultz gave Boston an 89-88 victory.

Fultz played well on both ends of the floor and showed why the 76ers took him with the top pick they acquired from Boston. He looked smooth on offense, playing both lead and off guard, but his defensive work may have been most impressive. Fultz finished with three blocks, including a weakside swat of Kadeem Allen that had the crowd gasping. He had 17 points on 6-for-16 shooting, including two 3-pointers

Tatum was not overshadowed as the 6-foot-8, 208-pounder showed the versatility he’s known for and knocked down a pull-up midrange jumper with 5.7 seconds left. Fultz’s shot was blocked on the other end to end the game.

Tatum, the No. 3 pick, may have had the most awe-inspiring play of the night with a thundering, one-hand tomahawk dunk on Kaleb Tarczewski. He posted 21 points, seven rebounds, three assists and five steals.

“He kind of has this old soul,” Celtics coach Jerome Allen said. “This pro-like approach, doesn’t get rattled, doesn’t show a lot of emotions. Just kind of plays at his pace like he’s been here before.

“We can say it’s only Summer League, but he just made plays. He turned the corner, he was aggressive, he shared the ball, he defended and rebounded.”

While all eyes were on the two rookies, 2016 No. 3 overall pick Jaylen Brown was hands down the best player on the floor and looked like he had no business playing Summer League games. He finished with 29 points.

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LEAF EXCELS

Indiana Pacers first-round draft pick T. J. Leaf showed that his impressive debut wasn’t a fluke by following that performance up with an even better second game in Orlando.

Leaf improved on Sunday’s 12-point debut by scoring 19 points and grabbing 10 rebounds to help lead the Pacers to an 86-83 win over Miami to move them to 3-0. The 6-foot-10 forward from UCLA, who missed the Summer League opener on Saturday to attend a family wedding, showed some versatility with scoring on the perimeter and in the paint while converting 9 of 17 from the field in 28 minutes.

In a game that was easily the most heavily contested of the day in Orlando, it was the play of Joe Young, Chris Johnson and Leaf that allowed the Pacers to pull away in the final five minutes. There were four lead changes and the score was tied nine times. The Heat pulled within 84-81 with 23 seconds remaining before Young converted two free throws.

Young finished with 17 points and six assists, while Jarnell Stokes added 14 points and Johnson contributed 12.

Miami’s first-round pick Bam Adebayo led all scorers with 29 points and grabbed 11 rebounds in nearly 32 minutes.

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COMEBACK MAVS

The Dallas Mavericks found themselves quickly in a 17-point hole against Orlando during Monday’s game.

But the double-digit deficit wasn’t much of a problem as the Mavericks recovered in the second quarter and then separated in the third quarter on the way to an 86-76 win. Dallas improved to 2-0.

Guard Dwight Buycks led the Mavericks with 20 points, while forward Brandon Paul added 18.

The Magic appeared dominant in the first quarter with No. 6 overall pick Jonathan Isaac leading the way with eight points. But Isaac injured hurt his hip on a collision in the second quarter and didn’t return in the second half to take away Orlando’s scoring punch.

Point guard Marcus Georges-Hunt, who was called up from the D-League by the Magic in April, contributed 20 points but he had little help. Shooting guard Patricio Garino, who was also called up from the D-League in April, converted just 1 of 4 from the field and went 0 for 2 from 3-point range to contribute four points in 19 ½ minutes.

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NTILIKINA UPDATE

Knicks rookie Frank Ntilikina has missed the first three games with a knee injury.

It’s unclear whether the No. 8 overall pick will participate in Summer League play. Knicks coach Jeff Hornacek did not talk to reporters following the game.

New York dropped to 0-3 after Monday’s 99-87 loss to Oklahoma City.

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AP Sports Writer Kareem Copeland in Salt Lake City contributed to this report.

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