ST. PETERSBURG -- In an up-and-down season so far, the Mariners have had one impressive constant: beating the Rays.

That trend continued Monday night at Tropicana Field, when the Mariners cooled off the Rays with an 8-2 win in the opener of a three-game series, handing a lopsided defeat to a previously red-hot team that came in tied for the American League’s best record and was coming off a sweep of the Red Sox.

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What gives? The Mariners are 5-0 against the Rays in 2021, winning every way possible, whether it’s solid pitching, timely hitting, late-inning rallies or the start-to-finish domination that occurred Monday night.

“We’ve been on the right side of five games so far,’’ Mariners manager Scott Servais said. “I hope we can figure it out a couple more times, then get out of Dodge. We’ve played well against them. I can’t really explain it.’’

Rays manager Kevin Cash and his team can’t explain it, either.

“Seattle, they’ve kind of had our number,’’ Cash said. “We’re going to have to make some adjustments moving forward. They’ve played really well against us.’’

The Mariners, who posted a four-game sweep of the Rays (with three walk-off wins) in Seattle on June 17-20, came to St. Petersburg on an emotional low. They were twice beaten by the last-place Rangers on walk-off home runs. Meanwhile, the newly crowned first-place Rays were flying high.

“I don’t know what it is about the Mariners, but they’ve just given us a lot of trouble over the years,’’ Rays center fielder Kevin Kiermaier said. “They’ve got really good players over there, and they had one big inning [five runs in the third] and it’s tough to come back.’’

But the Mariners capitalized on 6 2/3 strong innings from right-hander Chris Flexen (10-5), who allowed two runs on seven hits while walking two and striking out six on 113 pitches.

“I really had a good curveball tonight … that was my X-factor,’’ Flexen said. “They’re a really good team, but we had a great game plan. We kept attacking, kept putting our foot down on the gas.’’

The Mariners compiled a five-run third inning, in which the first seven batters reached base, four with singles. It was their biggest inning since June 9, when they scored five 11th-inning runs against the Tigers. Seattle made life miserable for Rays starter Michael Wacha (2-3), who needed 42 pitches to escape the frame.

“We hit six or seven balls on the screws [against the Rangers], but they all got caught,’’ Servais said. “Tonight, the balls dropped. It starts with the guy on the bump. You put starting pitching together with a lot of really good at-bats … and it’s a good way to start this series.’’

First baseman Ty France finished a triple short of the cycle, going 3-for-5 with a solo homer in the seventh. He said the Mariners weren’t fazed by those two walk-off defeats to the Rangers, and they showed it in Monday’s turnaround victory.

“I don’t think we had too rough of a weekend, because we played good baseball,’’ France said. “It’s how the game goes. You’re going to have highs and lows. Tonight, we got some hits and took advantage of that. The Rays are a hot team right now. To play the way we did, that was huge.’’