Asked why he has had his best games of the season against the Somerset Patriots, Portland Sea Dogs pitcher Jay Groome grinned ear-to-ear. It’s all about the rivalry that runs from the big leagues through the entire Boston Red Sox organization.

“That’s the Yankees. That’s our rival. I always go out there with a chip on my shoulder, pretend like I’m pitching against the New York Yankees,” Groome said Tuesday afternoon after throwing six strong innings in a 2-1 win over Somerset, the Yankees’ Double-A affiliate, in front of 6,996 fans at Hadlock Field.

The 6-foot-6, 265-pound left-hander allowed four hits and struck out nine. The one run Groome allowed was unearned, and he lowered his ERA from 3.45 to 2.89.

“When he’s going (well), he has what you see today,” said Sea Dogs Manager Chad Epperson. “He’s being aggressive in the strike zone with all his pitches. Being able to work ahead (in the count) obviously puts him at an advantage against the opposition. Whether it’s the breaking ball or whether it’s the fastball, being able to locate it and he’s working ahead of hitters, and that’s huge for him.”

It’s been an up-and-down season for Groome, 23, but lately mostly up. In each of his last three starts, Groome allowed two or fewer runs. His nine strikeouts on Tuesday were one shy of his season high, when he struck out 10 in an April 30 start at Somerset.

When he’s on, as he was Tuesday, Groome has command of all three of his pitches — fastball, changeup, and curve — and can throw them for strikes in any count. Fifty-four of his 88 pitches Tuesday were strikes, and five of his nine strikeouts were looking.

Boston’s first round-draft pick  in 2016, Groome began the game with back-to-back strikeouts. First, he fanned Somerset leadoff hitter Anthony Volpe, the Yankees’ No. 1 prospect, on a high 92 mph fastball. Then, Groome got Elijah Dunham looking at a changeup on the outside corner.

“I have a plan every time I go out there to just attack the strike zone. The last three or four outings I’ve just been really diligent with how my stuff plays in the zone. When I’m at my best, I’m throwing everything in the zone. I know I have good enough stuff to get swings and misses in the zone,” Groome said.

“Everything was working today. I got a bunch of swings and misses on my changeup. That’s always good to see. It’s not always a swing and miss pitch for me. I’m going to keep using it. As you saw today, it’s got some life to it.  Hopefully I can keeping going and keep turning these quality starts.”

Groome allowed an unearned run in the fourth. Brandon Lockridge doubled to lead off the inning and took third base on an error. Groome then walked Jeisson Rosario. He struck out Chad Bell, then got a grounder to second from Michael Beltre, who beat out the relay throw from second to stay out of the double play and allow Lockridge to score.

“He had the double play ball. It wasn’t the best of turns for us, but he got what he wanted,” Epperson said.

Two starts are a reminder to Groome that he has to pound the strike zone. On April 19 against Binghamton, his second start of the season, Groome lasted just 1/3 inning, walking three and allowing three runs. On May 6 versus Reading, Groome walked four and allowed seven hits in 3 2/3 innings, surrendering four earned runs.

“I get myself in trouble when I just walk people,” Groome said. “I know I have all the physical attributes. I’m 6-6, 265 when I roll out of bed. It’s mainly the mindset. Getting ahead and staying ahead of hitters. I know I get into trouble when I’m behind in the count.”

Added Epperson: “He knows when he’s out there kicking his own ass. I don’t see any difference in confidence. You just know when he’s had his rough outings, he’s not been able to execute pitch count in the strike zone, have a feel for all his pitches.”