CHICAGO – In what has already been a stellar season for Payton Pritchard, he continues to show his value to the Celtics when they need his skills most, Friday night at the United Center.

As the Celtics clung to their NBA Cup wild-card hopes, they traded baskets with the motivated Bulls, who needed a win to clinch Group C and secure a spot in the quarterfinals. Pritchard completely changed the game with his shooting technique, his drive and his tenacity.

He stunned the crowd with 19 points in the first 6:35 of the final period, helping the Celtics take control of a run game and claim a 138-129 victory.

The Celtics finished 3-1 in Group C but finished in second place due to their 1-point loss to the first-place Hawks, who needed a Boston win over the Bulls to prevail. After Pritchard’s shot, the Celtics used some key balls from Kristaps Porzingis, Jayson Tatum and Jrue Holiday to hold off the feisty Bulls.

Tatum led Boston with 35 points, while Jaylen Brown and Porzingis added 21 points each. Pritchard finished the game with 29 on 10-for-16 shooting (7-of-11 on 3-pointers). The Celtics, trying to clinch the wild-card spot, played until the final seconds, with Sam Hauser hitting a 3-pointer at the buzzer to take the final lead.

It was clear in the third quarter that Tatum (14 rebounds) had to get the better of his teammates, and he responded after the Celtics again started slowly early in the second half and trailed 80-72. Tatum scored 13 of the Celtics’ next 24 points by relentlessly attacking the basket and also relying on his 3-point shooting.

The Celtics needed every point because there was little they could do with Nikola Vucevic, who scored 29 points in the first three periods, including five 3-pointers. The Bulls also received contributions from some unusual sources like Torrey Craig, who took Kristaps Porzingis off the dribble and hit a step-back 3-pointer.

It was a classic shootout and the Celtics were tied at 96 points early in the fourth quarter.

About an hour before tipoff, the league released four scenarios in which the Celtics could secure the Eastern Conference wild-card spot in the NBA Cup with a win on Friday. In all scenarios, the Celtics had to win by at least 16 points, and they acted like they were trying to secure last place.

Behind Jaylen Brown’s three three-pointers, the Celtics took a 23-12 lead and overwhelmed the Bulls with long-range shots. The lead later grew to 36-23 after a 3-pointer by Tatum, and it appeared to be the same offense that blew up the Clippers on their trip to Chicago on Monday.

But after coach Joe Mazzulla pulled Tatum out at the start of the second period and played most of the second session, the lead disappeared. The threes stopped falling and the Bulls bench thrived against their opponents. Craig started the period with two three-pointers. Long-traveled Talen Horton-Tucker scored seven straight points as the Bulls completely erased a 13-point deficit.

Lonzo Ball, out the last two years with knee problems, hit a layup high off the glass and Zach LaVine followed with a baseline jumper to give the Bulls a 55-46 lead.

Behind Tatum, the Celtics responded with a 14-0 run to retake the lead, and Derrick White’s corner 3-pointer put Boston up 60-55. The craziness continued as Vucevic, who had made four or more three-pointers in three games this season, scored four goals in 93 seconds.

Tatum’s 3-pointer tied the game at 67 at halftime, but the Celtics scored 37 points on 60 percent shooting in the second period, again plagued by defensive inconsistency. Tatum led the Celtics with 19 points and White added 16, but the Chicago bench dominated the half with 23.

Gary Washburn is a Globe columnist. He can be reached at [email protected]. Follow him @GwashburnGlobe.