OAKLAND -- Frankie Montas strengthened his All-Star case with another dominant performance, but it was Matt Chapman who may ultimately garner a boost in the voting.

Down to their final out in a ninth inning that began with the A’s facing a three-run deficit, Chapman completed an exhilarating comeback as he blasted a three-run home run off Rays reliever Diego Castillo for a walk-off 5-4 win at the Coliseum.

• Box score

It was not an easy at-bat for Chapman, with Castillo pumping a 99 mph fastball that appeared to have very little control. Taking a first-pitch fastball for a ball that missed high, Chapman said he had a pretty good feeling that a slider was coming next. Sure enough, the slider was thrown down in the strike zone and Chapman clubbed it over the wall in left for his 17th homer of the season.

“Definitely not trying to hit a homer there, just put a good swing and get those two runs in,” Chapman said. “I’m still kind of shaking from it. That was so much fun.”

The rally began with walks by Robbie Grossman and Jurickson Profar, with Grossman later scoring on a single by Marcus Semien, who pushed his MLB-leading hitting streak to 17 games. Semien and Profar advanced to second and third base on an error after Semien’s hit, which brought Chapman to the plate.

“Once I got on deck, I knew I was getting up,” Chapman said. “Marcus has been carrying our team. He’s playing unbelievable. To go up against a guy with great stuff and go right back up the middle gave me confidence.”

The comeback erased the dejection felt inside the A’s dugout after closer Blake Treinen was unable to preserve a 1-1 tie, tagged for three runs in the top of the ninth to give Tampa Bay a short-lived lead.

“Just a roller coaster of emotions,” said Ryan Christenson, who served as acting A’s manager on Thursday with Bob Melvin away from the club. “This is a team that we’re fighting with for the Wild Card position right now this early in the season and a very good team we respect. To go down three runs was a kick to the gut, but this is just a sign of what our team has been showing the last few years. They never quit.”

There are times when your team leader needs to come through in special moments, and though he’s only 26 in his second full big league season, this is a role Chapman has taken on. Thursday marked the second game he’s ended with a homer this year.

“His maturation here at the big league level is impressive,” Christenson said. “He’s able to slow the game down. He wants the big at-bats and he comes through in big at-bats. When your leader comes through like that, it sets the tone for the rest of the team to follow.”

Knowing the possible playoff implications this series could have later in the year -- Tampa Bay currently holds the first AL Wild Card spot -- Chapman said this victory had a little more importance behind it.

“That’s a big momentum boost for us,” Chapman said. “If you want to be a playoff team, you have to be able to come back like that.”

Montas duels Morton

Montas was denied what would’ve been an MLB-leading 10th win despite one of his most dominant outings in a season that has seen him produce many.

The marquee matchup between potential All-Stars in Montas and Charlie Morton lived up to its billing. The two-right handers were deadlocked in a battle early as they exchanged zeros on the scoreboard until Morton budged in the fifth inning by surrendering a solo home run to Profar.

“I like the competition. I like to pitch against good pitchers,” Montas said. “Morton is one of the best right now and I like a challenge. If he’s putting zeroes on the scoreboard, I want to put up zeros, too.”

Montas was unable to make the one-run cushion stick as he gave up a solo homer to Tommy Pham the next inning, breaking a streak of 10 consecutive batters retired, but he quickly brushed that off and retired the next eight batters in a row to end his night. Displaying perhaps the best command he’s had all year, Montas cruised through eight innings, allowing just one run on four hits and no walks, striking out nine on just 93 pitches.

The A’s have grown accustomed to Montas’ success -- he lowered his ERA to 2.70 over 15 starts -- but his last three outings have been particularly off the charts. Montas has compiled a 1.80 ERA in his last three appearances, racking up 28 strikeouts and just two walks.

“He’s been one of our best pitchers all year,” Christenson said. “These starting pitchers are putting together some good starts consecutively here and he was in complete control of that game.”