SOUTH PORTLAND — This was a bizarre game from the start.

Portland didn’t take a shot from its set offense for the first 13 minutes in its boys’ basketball game Tuesday night against South Portland.

After his team lost 20-16 in overtime, its second straight loss after winning its first 15 games, Coach Joe Russo said his team wasn’t stalling, just being selective.

And South Portland seemed so excited when it did get possession that it rushed down the floor for a quick shot or a turnover.

There was frustration on both sides. In overtime, though, South Portland summoned enough poise to outlast Portland’s tactics and avenge a two-point loss to the Bulldogs on Dec. 18.

“This win guarantees us second in the rankings,” said South Portland Coach Phil Conley.

Russo said two things dictated the Bulldogs’ style: “We were concerned about their height and we wanted to bring them out of their zone,” he said. “But they didn’t come out of their zone.”

Portland, top-ranked and the defending Class A state champ, had its two-year unbeaten streak stopped at 37 games Saturday with a 55-44 loss to Cheverus. That was another reason Russo decided to slow things down.

“We rushed our shots against Cheverus,” he said.

Portland almost won anyway. The Bulldogs led 11-8 entering the fourth quarter and Amir Moss missed a jumper at the end of regulation that would have won it.

In overtime the Riots (14-3) got the jump, scoring off the tap on Ruay Bol’s layup to make it 15-13. Jaren Muller then hit two from the line to make it a four-point swing. The Riots missed their next five free throws.

But the Bulldogs, who could no longer be overly selective on their shots, were having problems, missing two inside shots, then throwing the ball away after two misses from the line by the Riots.

Jack Fiorini scored inside for South Portland to make it 19-13. Liam Densmore hit a 3-pointer from the corner with 23 seconds left to cut the Riots’ lead to 19-16, but Muller hit 1 of 2 from the line with 8.4 seconds left.

“Portland used a tactic we haven’t seen before,” said Conley. “My players hung tough. They stayed the course defensively. The key in overtime is that we got the jump and we scored.”

Asked why he thought Portland used its strategy, Conley said: “We’re a big team and they wanted to take our bigs out of the game.”

Portland won the opening tap and passed the ball around outside, not looking to penetrate. Kam Harper of South Portland stole the ball at 2:40 but threw it away. The Bulldogs missed three straight shots off an earlier up-and-down the court flurry.

Muller’s layup accounted for the only first-quarter points.

Densmore took Portland’s first shot in the set offense with 3:05 left in the half. He hit a 3-pointer from the corner to make it 6-3.

The Riots led 7-3 at halftime but Portland used an 8-1 run in the third quarter for an 11-8 lead. Portland went ahead after a steal and layup by Stephen Alex with 5:44 in the third. Fiorini hit one from the line to tie it moments later. Densmore’s 3-pointer from the corner gave the Bulldogs the lead.

“We showed our poise in overtime and got the job done,” said Fiorini. “This is a huge win. It gives us a lot of confidence with the tournament ahead.”

Russo said he wasn’t upset with the loss or the low score.

“We’ve sown up first place,” he said. “I’m disappointed by the fact we didn’t capitalize on our chances. We had a chance to win this game.”