BOSTON -- The announced temperature at Fenway Park at first pitch was just 41 degrees.

While his teammates on the bench were wearing winter hats and parkas, starting pitcher Mitch Keller trotted to the mound in short sleeves and put together the best pitching performance by a Pirate in this young season, allowing one run over seven innings to lead his team to a 4-1 victory over the Red Sox on Wednesday afternoon in the series finale.

"I just never really wore sleeves growing up. I don't know,” the 27-year-old Iowa native explained. “I just kind of stay hot, get hot."

“Pitching was really good today. Mitch was outstanding,” said manager Derek Shelton. “We used a lot of our leverage guys and were a little short. For him to give us seven strong and finish was really important. Outstanding work from him.”

Over his first five innings, Keller only allowed two baserunners. After giving up a second-inning single to Christian Arroyo, the Pirates hurler retired the next 12 batters, striking out the side in the fifth.

Carlos Santana staked Keller and company a lead with a solo shot tucked inside Pesky's Pole to put the Pirates up, 1-0, in the fourth. The Bucs would pick up another run in the sixth and two more in the top of the seventh.

For a moment it looked as if all of Keller’s hard work was washed away in the bottom of the seventh, when pinch-hitter Reese McGuire hit what was initially called a three-run homer that would've tied the game at 4. But following a conference between the umpiring crew, the ball was called foul, and instant replay confirmed the call.

A few pitches later, on his 107th of the day, Keller reared back and fired a 96 mph fastball past McGuire to end the seventh and his start.

"Yeah. It was kind of crazy, the turn of events there. Went from a home run called on the field to a pretty quick -- well, it wasn't quick -- overturn,” said Keller. “I was watching the ball and was kind of amazed they called it fair. Just had to get back in there and make sure I finished the at-bat for us."

“I was a little surprised he called fair,” echoed Santana, who had one of the better angles on the ball from first base. “It’s a foul ball. Little bit tough. Big fly. Thank God it was foul and we won the game."

Keller, who had 13 quality starts last season, picked up his first win of the season and finished the day having allowed just one run on four hits and two walks while striking out seven. Of his 107 pitches, 78 were strikes, the most by a Pirates pitcher since Gerrit Cole did the same on April 24, 2015.

"I had no idea [I threw that many],” Keller said. "Just attacking the zone. The cutters to lefties were great. Sinkers were great to righties, too. Just filling it up, trying to throw as many strikes as possible. We had a good game plan and got in a rhythm there with [catcher Jason] Delay.”

With the win, the Pirates leave Boston with a sweep and head to Pittsburgh for their home opener on Friday with a 4-2 record after the first week.

“Everybody is doing good,” said Santana. “The energy is great. We have to keep it up. It’s a long season. ... We played hard this series. We have to continue doing it.”