George Mason’s Greg Calixte (33) celebrates a basket during the first half of the Patriots’ win over U-Mass. in the second round of the Atlantic 10 tournament Thursday at Capital One Arena. (Jonathan Newton/The Washington Post)

George Mason had faced many high-anxiety situations in the waning days of this erratic basketball season, so when the Patriots found themselves locked in a tight Atlantic 10 second-round game with Massachusetts on Thursday, earlier lessons took hold.

A freshman sharpshooter swished a three-pointer and made life a little more difficult for a 31-point scorer. A rail-thin freshman from Minnesota made three of four free throws. Two freshmen rebounded with a fury.

And after junior guard Otis Livingston II, a downright grizzled veteran on a team with no scholarship seniors, sealed the 80-75 victory with two free throws, the fifth-seeded Patriots (16-16) were on to the quarterfinals.

The Patriots will play No. 4 Saint Joseph’s (15-15) at 2:30 p.m. Friday.

Mason’s opponents 24 hours apart at Capital One Arena are fitting: GMU’s two regular season victories against U-Mass. had come in overtime and the two triumphs over St. Joe’s were buzzer-beaters.

“With this league — always, historically, but certainly this year — you know every single game is a grind-it-out game,” Patriots Coach Dave Paulsen said. “They are all coming down to the wire and they are all going to coming down to one or two possessions. What we try to tell our guys is . . . every possession has meaning and has significance.”

The Patriots made almost all of them count. In the last 8½ minutes, they scored on 12 of 14 possessions and, despite their youthful lineup, did not commit a turnover in nosing out the No. 13 Minutemen (13-20).

“It’s just something that we’re comfortable with, having close games in the regular season,” said Livingston, who, despite shooting poorly from the field, scored 21 points. He made all 14 free throw attempts, the third-highest count without a miss in program history.

But with Livingston and the other two starting guards off the mark, Paulsen received key contributions from his youngsters. Javon Greene came off the bench to make three of four three-pointers and score 11 points. Reserve AJ Wilson posted 11 points and nine rebounds (seven offensive). Starters Goanar Mar and Greg Calixte combined for 24 points, and Calixte added seven rebounds (five offensive) and three steals.

Greene was also tasked with guarding Luwane Pipkins most of the game. The 5-foot-11 sophomore was unstoppable until the closing moments. With Greene harassing him, Pipkins missed three shots and, with the Patriots ahead by three with four seconds left, the Minuteman pushed the ball upcourt . . . and then pushed off on Greene.

“I just had to guard him and not let him score easy buckets — make all his buckets tough,” Greene said.

Pipkins, the A-10’s second-leading scorer at 20.9 points, produced at least 30 for the sixth time this season. He made 9 of 15 shots, including 5 of 8 three-pointers, and added five rebounds and four assists.

Pipkins and Carl Pierre combined for 30 points in the first half as the Minutemen made seven of their first eight three-point tries, shot 62 percent overall and built a 14-point lead.

“We had a little too much panic in our eyes,” Paulsen said of a first-half timeout. “We have to have a greater sense of urgency on the defensive end.”

Mar led the comeback with a three-pointer, a three-point play and four additional points late in the half. Tied at intermission, the Patriots fell behind by eight in the second half. This time, Wilson and Greene sparked the resurgence.

Mason finished with 20 offensive rebounds, won the overall rebounding battle 37-29 and scored 26 second-chance points.

Paulsen praised Greene and Mar, then said, “the real bonus was the play of Wilson and Calixte. . . . The [four] have really embraced working hard and all they care about is their teammates. It’s fun to watch their maturation.”

● VIRGINIA COMMONWEALTH 77, DAYTON 72: The No. 8 Rams (18-14) ended the game on a 12-2 run to defeat the No. 9 Flyers (14-17) and set up a showdown with top-seeded Rhode Island (23-6) at noon Friday.

Rhode Island won the regular season meeting by 13 points on Feb. 2 in Richmond.

Needing the league title to extend a run of NCAA tournament appearances to eight, VCU turned a 70-65 deficit into a 72-70 lead, capped by Jonathan Williams’s acrobatic shot in the lane with 58 seconds to go.

Three free throws by Williams (14 points) all but sealed the outcome for the Rams, who made four big defensive stands down the stretch.

Justin Tillman led VCU with 15 points, 10 rebounds and three blocked shots. Trey Landers had 19 points and 10 rebounds and Josh Cunningham added 17 and 11 for the Flyers.