Kevin Youkilis watches the flight of the ball as it goes over the Green Monster in the bottom of the 13th inning yesterday, giving the Red Sox a 5-3 victory over the Cardinals in the series finale at Fenway Park.

The Providence Journal / Mary Murphy

BOSTON — Kevin Youkilis thought the 37,085 fans at Fenway Park yesterday were uncharacteristically quiet. He perhaps thought some Boston fans were still a little groggy from the Celtics winning the NBA championship last week and didn’t have enough in the tank to last through a 13-inning, four-hour marathon.

He changed that with one swing of the bat.

The crowd, and his Red Sox teammates, erupted when Youkilis crushed a two-run, walk-off home run in the bottom of the 13th inning to give Boston a 5-3 victory over the St. Louis Cardinals. It was the first time Youkilis had hit a homer to win the game as he deposited a fastball from St. Louis reliever Mike Parisi into the Monster Seats.

“It was really quiet, but it was good to get a lot of cheering going on right there,” Youkilis said. “It’s always good when you get to run around the bases . . . I knew I hit it really well, but you never know here with left-center field and the wind. It felt good off the bat and I was hoping Mike Lowell was running as hard as he could in case it didn’t get out.”

The reason the Sox needed extra frames was because closer Jonathan Papelbon blew his fourth save opportunity of the season in the bottom of the ninth inning. Boston held a 3-2 lead when he issued a two-out walk to the Cards’ Chris Duncan before Adam Kennedy provided a game-tying RBI double.

It’s not like the Red Sox didn’t have chances to end it before the 13th, however. Boston led off the 10th, 11th and 12th innings with doubles, but the runners were stranded. Lowell led off the 13th with a single off the Monster before Youkilis provided the heroics.

It has been a long couple of days for the Red Sox, but the weekend series ended in fine fashion as they avoided a sweep by the Cardinals.

“It was wonderful,” said Sox manager Terry Francona, with a sigh of relief. “When you get into one of those games, at least when you’re playing at home, you know you have the last at-bat. We got Pap in the game and felt good. They tied it and then we kept the pressure on, but we couldn’t get that run across. Some days that’s what it takes, for someone to run the ball out of the ballpark. I know it’s a heck of a lot better hearing the music than coming in frustrated after a long day.”

Anything that could possibly happen in a ball game nearly did yesterday. There was a rain delay. Both starting pitchers were outstanding. Both defenses were sometimes spectacular and lousy at other times. There was a dramatic collision at the plate with the Cardinals’ Chris Duncan and Red Sox catcher Jason Varitek in the top of the 13th inning, which Varitek won by making the putout.

Basically, there was everything and then some. The final piece was Youkilis’ second walk-off hit of the season.

“It was good to get it over with,” Youkilis said. “It was getting a little tiring out there. It was good just to get out of here with a win and not get swept. We had plenty of opportunities, and it was one of those crazy days, but to get out of here with a win was huge for us.”

It was a huge moment for Youkilis, too.

Earlier this month, Youkilis and teammate Manny Ramirez got into a scrum in the dugout during a game. Reportedly, the Red Sox held a team meeting to discuss with Youkilis his routine of throwing of helmets and bats when things didn’t go his way, and when he displayed his displeasure again during that game, against the Rays, Ramirez said something and took a swing at Youkilis.

The following day both players called it a misunderstanding and have moved on.

No doubt the Red Sox’ players were pleased with Youkilis’ fist-pumping action yesterday when the ball landed in the Monster Seats.

“He always puts together a good at-bat,” said Lowell. “You feel pretty good when he’s at the plate. You don’t expect a home run every time, but it was a nice way to captivate the moment.”

jmcdonal@projo.com