NEW YORK — Brett Gardner smiled through the fog left from a gaudy celebration in the New York Yankees clubhouse, part of the new postgame routine that includes the razzle-dazzle of a smoke machine and strobe lights.

“I think the idea was to play it after we win big games,” he said.

This was as big as an April game gets.

Gardner hit a grand slam off Ryan Brasier in the seventh inning for his 100th career home run, and the Yankees rallied to beat the Boston Red Sox 5-3 Wednesday night for a two-game sweep of the reeling World Series champions.

“About time. I think he was just waiting for the right moment,” teammate Aaron Judge said with a mischievous grin.

J.D. Martinez hit a first-inning home run off J.A. Happ, just like he did in the AL Division Series opener last October, and Christian Vázquez’s two-run homer in the second built a 3-0 lead. The Yankees were on the verge of failing to win any of their first four home series for the first time since 1913, according to the Elias Sports Bureau.

Clint Frazier started the comeback with an RBI double in the fourth against former Yankee Nathan Eovaldi, and New York loaded the bases in the seventh against Brandon Workman on Frazier’s single and walks to Mike Tauchman and Austin Romine.

Brasier got ahead 0-2 as Gardner took a slider and fouled off another. Brasier left a fastball up that Gardner drove 364 feet over the right-field scoreboard.

“It was chaos. The Stadium was rocking. Our dugout was going crazy,” Judge said.

Tommy Kahnle (1-0) got two outs for the win, Adam Ottavino retired Eduardo Núñez on a bases-loaded flyout that ended the eighth and Aroldis Chapman got three straight outs for his third save as the Yankees improved to 8-9, including 5-6 at home.

Gardner’s hit led to the postgame vapor, technology installed after general manager Brian Cashman had heard about young players on other teams embracing flashier jubilation.

“We kind of need a little kick start,” Gardner said. “Obviously, these first couple of weeks we’ve been dragging.”

Frazier had three hits and raised his average to .333 in 45 at-bats with 12 RBIs. He proclaimed before spring training he hoped to take away Gardner’s job — some day.

“He’s been like a dad at the field for me,” the 24-year-old Frazier said. “He’s always led me in the right direction. He’s let me be myself around him regardless of how ridiculous at times I’ve acted. So he’s a great guy to come in here and be good to the young guys.”

Boston dropped to an AL-worst 6-13, the poorest start for a World Series champion since the 1998 Florida Marlins opened 5-14. The Red Sox did not lose their 13th game last year until May 14 and face daunting math: No team has lost 12 or more of its first 18 games and reached the playoffs since the 2002 Anaheim Angels, who won the World Series.

“I made a bad pitch on a good count for me and it bit me in the butt,” Brasier said.

Boston dropped eight games behind AL East-leading Tampa Bay.

“We always have a sense of urgency,” Red Sox manager Alex Cora said. “We need to get better in all aspects of the games.”

GLOVE GEM — oston 1B Steve Pearce saved two runs, diving for a backhand snag of Gio Urshela’s two-out chopper with runners on second and third in the fourth, then throwing to Eovadli covering first.

STARTERS — Eovaldi allowed an unearned run and three hits in six innings, while Happ gave up three runs and six hits in 6 1/3 innings. Happ has allowed six home runs in 18 2/3 innings over four starts.

TRAINER’S ROOM — Red Sox: 2B Dustin Pedroia left in the middle of the second because of left knee discomfort he felt while fouling off a pitch on his first swing of the top half. He was to stay behind in New York and be examined Thursday. “He was down. I haven’t seen him like that,” Cora said. … OF Andrew Benintendi missed his third straight game after fouling a pitch off his right foot Sunday. Cora said he expects him to start Friday. … Utilityman Brock Holt (scratched cornea) was to see an eye doctor again and perhaps get a new lens. He did not play for Triple-A Pawtucket on Wednesday.

UP NEXT — Red Sox: LHP Eduardo Rodríguez (1-2) starts Friday, beginning a stretch of 20 games in 20 days for Boston.