TORONTO — It had been a relatively quiet start to May for Kyle Seager. Traditionally in his career, his sour starts in April bring May power and production. But coming into Thursday night’s series finale against the Blue Jays, he was hitting just .230 (7 for 30) with two doubles and an RBI eight games into the new month.

But that May be changing.

Seager blasted a pair of homers, including a first-inning grand slam, to lead the Mariners’ offensive explosion that included 17 hits and four homers in a decisive 9-3 pummeling of the Blue Jays.

“Kyle really had a great night,” manager Scott Servais said. “We are into May now. We didn’t talk about April with Seags. Hopefully he can get hot and sustain a hot streak for a long time. He’s a big part of our offense.”

Seattle improved to 21-15 and picked up its eighth series win after dropping its previous series against the Angels. The Mariners improved to 5-0-2 in road series this season.

“We came in here and won the series,” Servais said. “We had the no-hitter on opening night, and I felt like we let last night’s game get away from us. Just keep it rolling. We go into Detroit and then have the one game in Minnesota. You want to finish strong.”

The Mariners beat up on one-time teammate J.A. Happ, knocking around the veteran left-hander for seven runs on 10 hits in just 31/3 innings. Happ came into the game with a 4-2 record and 3.67 ERA while not allowing more than four runs in any start.

Seager plated four runs with one swing.

With two outs, Ryon Healy worked an eight-pitch walk to load the bases and bring Seager to the plate.

In this situation, the instant analysis would be advantage Happ. While he hadn’t faced many lefties this season, Happ came into the game holding lefties to a .122 batting average. Of the five hits allowed in 41 at-bats, just two were extra-base hits, both doubles.

And yet, Seager isn’t the typical lefty that is overwhelmed by left-handed pitchers. He had two homers in his career against Happ and has hit 32 homers off left-handed pitchers the last three seasons.

“I think those were a few years ago,” Seager said of his previous Happ homers. “I think the last few years he’s kind of had my number.”

Indeed, Seager’s two homers came in 2014. In his previous seven at-bats against Happ, he was 0 for 7 with two strikeouts.

After falling behind 0-2, Seager refused to chase a high fastball out of the zone as Happ searched for strike three. The next pitch was a 94 mph fastball down the middle. Seager redirected the mistake over the wall in right-center for his fourth career grand slam.

Was he looking for a certain pitch?

“I got a fastball and I was looking to not strike out, so it worked out all right,” he said. “When you’ve got bases loaded and two outs, you are just trying to make sure you put the ball in play and make something happen. Fortunately for me, it had enough to get out.”

It was a miserable first inning for Happ. He faced all nine Mariners batters and threw 37 pitches.

Seattle tacked on one run each of the next four innings, allowing the Mariners to brush off a two-run homer by Russell Martin in the second inning.

Healy led off the third inning with his sixth homer of the season, smashing a solo blast off the facade of the upper deck in deep left-center to make it 5-2.

The Mariners loaded the bases again off Happ in the fourth inning. Robinson Cano scored the only run on a sacrifice fly to left that ended Happ’s night.

While the grand slam gets the notoriety, Seager’s solo smash off Jake Petricka to start the fifth inning might have been more impressive. Not known to have consistent power to that part of the park, Seager sent a fly ball over the wall in dead center.

“Those are good,” he said. “You can have a pull-side homer with a bad swing. Those happen all the time and you love them, don’t get me wrong. But that doesn’t mean you are necessarily in a better position. The ball to center is a really good sign for me. It was a clean swing for me. It’s something to build off.”

All that run support was plenty for Seattle starter Mike Leake. The veteran right-hander got eight ground-ball outs, struck out six batters and allowed just the two runs on six hits while issuing just one walk in seven innings.

“The key was 21 out of 26 first-pitch strikes tonight,” Servais said. “When he’s getting that first pitch in there, he can work off his sinker and has the good breaking stuff.”

In his past three outings, Leake had been behind in about 70 percent of the counts.

“It’s something I needed to get back going and just carry it on to the next outing,” he said. “It puts the other team on their heels. I’m trying to make it easier on myself and attack guys better.”

The Mariners’ final run of the night might have been the most impressive. On a night filled with long homers, Mike Zunino’s solo homer off the neon Budweiser sign hanging off the front of the third deck in deep left-center might have been the best — officially 431 feet.