The Rays’ Willy Aybar, right, gets past Red Sox catcher Jason Varitek and scores on a fifth-inning double by Carlos Pena during last night’s 5-4 victory by Tampa Bay in St. Petersburg, Fla.

AP / Chris O’Meara

ST. PETERSBURG, Fla. — For one night, at least, this looked like some sort of baseball role reversal.

There were the upstart Tampa Bay Rays, buoyed by the twin novelties of their first-place standing and a partisan home crowd actually rooting (mostly) for the home team. And there were the Red Sox, full of stars and flush with accomplishment, but struggling with the young starting pitcher who too often couldn’t throw the ball over the plate.

Cobble it all together, and the Rays seized both their moment and the momentum by holding off the Sox, 5-4, before a raucous Tropicana Field crowd last night, building on their slight lead in the American League East.

The Rays hold a 1 ½-game edge on the second-place Sox, the farthest the Sox have been out of the division’s top spot since June 2. The loss was the third straight for the Sox — all on the road — while Tampa’s victory was its 50th, making the Rays the fastest team ever to that number for a team that finished with the game’s worst record the year before.

In the 10 meetings between the two teams this season, the home team has held serve each time, with Tampa winning all four here and the Sox 6-0 at Fenway.

In a thrilling ninth, the Sox scored twice against closer Troy Percival and had the potential tying run at third.

The inning featured a bizarre double by Brandon Moss, on which the ball got caught up in the ballpark’s ring of catwalks before falling into right field for a double, scoring Mike Lowell and sending Kevin Youkilis to third.

“As much as I (complain) about these stadiums,” said Terry Francona, “it almost helped us win the game.”

But after a sacrifice fly by Jason Varitek delivered Youkilis and sent Moss to third, J.P. Howell relieved Troy Percival (hamstring) and got Julio Lugo to line directly at shortstop Jason Bartlett.

Justin Masterson took the loss, chiefly because of command issues. He walked five — giving him a total of nine free passes in the last two outings combined — including two after two were out in both the fourth and fifth.

Both times, the walks cost him in a big way.

After the Sox had erased a 1-0 Tampa lead — produced on Masterson’s first pitch of the game, which B.J. Upton hit out to center — in the top of the fourth, Masterson got Evan Longoria and Eric Hinske for the first two outs.

He then walked Dioner Navarro and paid dearly when Gabe Gross walloped a sinker that stayed up in the zone and drove it deep into the seats in right.

An inning later, it was more of the same. Masterson got the first two outs quickly, retiring Upton and Carl Crawford. But he walked Willie Aybar, who came around to score on a Carlos Pena double to right.

“After we tied the game,” said Masterson, 4-2, “I’ve got to keep those guys from scoring and I didn’t. Walks always come back to hurt you. I’m sure it’s probably, deep down, something (to do with focus). It’s not like I’m thinking about easing up. But I’ve got to continue to pound the strike zone — it’s nothing more complicated than that.”

“He’s young,” said Francona of his starter. “I could see at times, him losing the plate one-two-three. I think that’s part of maturing as a pitcher. He doesn’t have a lot of innings.”

“When he commands and when he changes speeds, he’s really good. And like a lot of pitchers, when he doesn’t, he can himself into a bind.”

James Shields, who started for Tampa when the team last met — the infamous brawl-filled June 5 meeting that still reverberates — retired the first 10 Sox hitters in a row before an infield single by Dustin Pedroia and a two-out walk to Manny Ramirez.

Mike Lowell then laced a single off Longoria’s glove along the third-base line and Pedroia scored from second.

But Shields got Youkilis to hit into an inning-ending fielder’s choice and the only other run he allowed was a solo homer to J.D. Drew in the sixth. It was Drew’s 12th homer of the month and the most by a Red Sox hitter in June since Jackie Jensen hit 14 in 1958.

A leadoff single by Lowell in the seventh and a walk to Moss by Trever Miller gave the Sox an opportunity. But Grant Balfour turned back the bottom two spots in the lineup and the Sox didn’t stir again until the ninth.

smcadam@projo.com