The sad ghost of Big East past walked the tunnels of Madison Square Garden Thursday night as St. John’s got set to take on long-ago rival, Georgetown. It shrugged as professional Georgetown foil Chris Mullin played nice with the Hoyas, sighed as St. John’s bumbled through an uninspired first half.

And then it came roaring back to life in the second half, as a spirited St. John’s — sparked by vintage Mullin — reignited a rivalry as old as the conference itself, turning aside No. 9 Georgetown, 74-73 in the first round of the tournament Wednesday night.

With 8:07 left, Georgetown’s L.J. Peak sustained a hard foul that sent several players falling toward the stands. Tempers flared, benches cleared, and at the end of it all, Mullin had to be held back as he approached the Hoya’s bench. Instead of taking on Patrick Ewing, like Mullin did in his college days, the coach stared down Georgetown coach John Thompson III . . . and assistant Patrick Ewing. Jr.

Mullin and Ewing Jr. earned technical fouls as the Garden crowd rose to its feet, shocked, delighted and very, very loud. St. John’s also was assessed a flagrant 1 for the hit on Peak (he made one of two shots.)

That noise and excitement carried St. John’s until the end, as they held on to the lead they took with 9:55 remaining in the game. Up by one, St. John’s Shamorie Ponds missed a layup to give Georgetown the ball with 6.5 seconds to go. Peak missed his driving layup and Marcus Derrickson flubbed the putback to end the game. The Red Storm didn’t score in the final three minutes.

After the game, Mullin and Ewing exchanged conciliatory words (a new Big East development.)

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“I asked him if he was going to beat me up like his father did,” Mullin said to laughter. “He said, ‘No, I love you,’ so I said OK . . . Close game like that both teams know if they lose their season was over. I think it was just competitive juices going.”

No. 8 St. John’s (14-18) moves on to play No. 1-seed Villanova, Thursday at noon. The Red Storm broke a six-game losing streak in the Big East Tournament; it’s the first time they reached the quarterfinals since 2011. Peak scored 24 points; Ponds scored 17 and Malik Ellison added 11 points, six assists and eight rebounds.

“Everyone was riled up,” Ponds said. “It was a physical game and I think we just came out, end of the first half, we set the tone . . . We just took over the game.”

In all, it was an unexpected turn given their flat start. With 16:33 left in the first half, Georgetown took off on a 13-1 run, erasing the Red Storm’s three-point lead en route to an 18-9 lead for the Hoyas. Derrickson scored six straight in that span, including a booming dunk with 15:34 left.

The Red Storm eventually drew to within two on Bashir Ahmed’s three with 11:22 left and managed to tie it four more times after that, until Ellison’s free throws with 1:12 left in the first half put the Red Storm up by two. St. John’s went into the break up 38-34, despite a sloppy first half where they shot only 10-for-33 from the field. It also was thanks to the Hoya’s even sloppier play. St. John’s went to the line 18 times in the first half, and made 15.

“We played tough,” Mullin said. “It was just an old school type of game.”

The Big East of old tends to agree.