Top-ranked McAuley moved a step closer to a fifth straight Class A title but may have paid a heavy price Wednesday night in beating fifth-seeded South Portland 39-34 at the Cross Insurance Arena.

Sharpshooter Sarah Clement reinjured her surgically repaired right knee with 9.6 seconds left in the first quarter. While she tried twice to return, she was obviously hobbled and is doubtful for Saturday’s 7 p.m. regional final against second-seeded Thornton Academy (18-2).

Jess Willerson led McAuley (19-1) with nine points. Maddie Hasson scored 14 for South Portland (15-6).

Thornton and McAuley split two games this season, each winning a low-scoring game at home. McAuley, the four-time defending Class A champion, has won 19 straight since losing at Thornton in the opener.

Clement, who missed her entire sophomore season recovering from ACL surgery, had knocked down two 3-pointers from the left wing when she pump-faked from the same position and went to the basket. As she went up for the shot she was fouled and fell awkwardly.

Clement is scheduled to have an MRI done Thursday.

“It threw us for a loop and what stinks is that was the best she’s looked all year,” McAuley Coach Bill Goodman said. “It really was a blow.”

McAuley led 23-14 at the half, getting good offensive looks down low from the 6-foot-3 Willerson while South Portland struggled for openings.

The third quarter was all defense; the teams combined for one point over the first 6:55.

“That’s why we’re where we are now, because of our defense,” Goodman said.

Ayla Tartre drew the job of sticking with Hasson. Tarte held Hasson in check and the Riots couldn’t find a consistent secondary source.

“She drives to the basket a lot and my job was just to face-guard her and when she did have the ball, just to drop off. It was really important that we always had help,” Tartre said.

McAuley got accidental offense when a deflected ball skipped off a foot and went right to Brooke Howard, who converted for a 26-14 lead.

South Portland scored its four third-quarter points in the final 26 seconds when Hasson followed her own miss and Meghan Graff hit a deep buzzer-beating 2-pointer.

“I guess you give them credit, they play great defense,” South Portland Coach Lynne Hasson said. “We didn’t get a lot of open looks. We knew coming in they are hard to attack because they’re great defensively.”

In the fourth quarter South Portland cut the lead to six five times but couldn’t get closer until Lydia Henderson banked in a 3-pointer with 30 seconds to play, making it 37-32.

Ally Tillotson followed with two free throws for McAuley. Earlier in the period Olivia Dalphonse, the only returning starter from last year’s championship team, made four straight free throws in 1-and-1 situations.

Graff hit a runner at the buzzer for the final margin.

“If you go down in the fourth quarter they’re a tough team to come back on,” Hasson said.