BOSTON - Several hours before game time Friday night, Joe Girardi said one pitch likely would tell him the kind of night it would be for Michael Pineda.

"His slider's been really good, and that's his key pitch," Girardi said. "When you talk about a pitcher's swing-and- miss pitch, if they don't have their No. 1 swing-and-miss pitch, starts become a little more difficult."

Pineda's slider looked just fine in the first inning, and so did the rest of the big righty's arsenal in the Yankees' 5-1 victory over the Red Sox in front of 37,984 at Fenway Park.

"My fastball was really good tonight and I threw a couple good changeups and my slider was good tonight," said Pineda (9-5), who allowed one run, seven hits and no walks with six strikeouts in 62/3 innings.

After a day off Thursday, Alex Rodriguez drove in two runs and had two hits, including his 17th homer, a solo shot over the Green Monster seats and onto Lansdowne Street in the first inning that made it 1-0. But he seemed to want to talk only about Pineda.

"Michael was huge," he said. "I thought he may have had his best stuff of the year tonight."

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In improving to 21-14 in June and July -- the American League's second-best record in that span -- the AL East-leading Yankees (47-39) for one night put the brakes on a turnaround for the Red Sox (41-46) and maintained their three-game lead over the Orioles.

Boston, which had won four straight and nine of 12, could have moved within 21/2 games of the Yankees with a sweep of the three-game series, but the Red Sox reverted to the form that put them in last place, committing two errors. Three of the Yankees' runs were unearned and all five were scored with two outs.

"I think it's big," Girardi said. "You come in here and win the first game and you try to win a series. It gives you a chance to win a series now."

Red Sox starter Clay Buchholz left the game after 31/3 innings with what the Red Sox called "right elbow tightness."

The righthander, who entered the game 7-6 with a 3.27 ERA and was 5-2 with a 1.99 ERA in his previous 10 games, was charged with three runs (one earned) and six hits.

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For Pineda, it was a third straight strong start after the June 22 train wreck against the Phillies in which he allowed eight runs and 11 hits in 31/3 innings. Pineda has thrown 1061/3 innings, behind only the 171 he threw in 2011, his rookie season. Injuries cost him 2012 and 2013 and part of last season.

"I'm feeling very strong. I feel great," he said. "The last three years I've had to take a break because of injuries."

The Red Sox gave the Yankees three runs in a crazy fourth, an inning in which Boston committed its two errors, lost its starter and walked home a run as Pineda was given a 4-0 cushion.

The Red Sox finally got on the board with one out in the fifth when Mookie Betts hit his 10th homer. Jacoby Ellsbury got the run back with a two-out RBI single in the eighth.

The victory guaranteed that the Yankees will enter the break in sole possession of first in the division, regardless of the next two games.

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"This group has a good feel to it," A-Rod said. "It's a hard-working group, it competes hard. I think the key to us finishing the first half and also continuing in the second half is to stay hungry and humble."