"Able to throw strikes, be in the zone, making progress," he said.
Masterson struck out four as the Red Sox stretched their Grapefruit League winning streak to five. In five spring innings, Masterson has yet to surrender a run.
Masterson allowed only one ball out of the infield. The Rays seemed baffled at times by his sinkers.
"It means they're not ready yet," Masterson said. "I'm getting lucky. For me, it's really about hitting spots and today was pretty good as we kind of moved around, stayed in the middle."
Red Sox manager John Farrell likes what he's seeing from Masterson not only in two games, but also in his bullpen throwing sessions.
"I think with each outing Justin is making, we're seeing more sink to his fastball," Farrell said. "The ability to make an adjustment more readily from pitch to pitch. And just a pitcher who is becoming more aggressive."
Pitching well is helping Masterson's confidence as opening day is now less than a month away.
"My confidence has been high because everything has been great," Masterson said.
Masterson looked so strong after three innings that it appeared he could have gone for a fourth inning."
"No thank you," Masterson said. "Three was enough."
Masterson, who split last year between Cleveland and St. Louis, is coming off a season in which he spent time on the disabled list with right knee inflammation. He's also dealt with an oblique issue.
Farrell has observed a healthy Masterson in camp.
"But with each bullpen, each outing that he makes I think he's gaining confidence," Farrell said.
Another plus for Boston was the return of right fielder Shane Victorino after sitting out the past three games to rest his legs.
"He brings tons of energy. Brings a lot of the crowd and Vic is a great player," Masterson said.
"Look at him today, started the game off with a walk. Great eye right there. He's a guy who can make stuff happen. When he's feeling good he'll be running around, catching balls in the outfield. He'll just be a pest. That's what makes him so great. He's just a pest at the plate," he said.
New Red Sox left fielder Hanley Ramirez hit an RBI double.
Jackie Bradley Jr., competing for playing time in the Boston outfield, doubled and singled with a walk. He's 5 for 13 this spring.
Rays: Jake Odorizzi starts against the Twins.
Red Sox: Joe Kelly starts against the Yankees in Tampa, Florida.