SEATTLE -- Chris Smith was even better than advertised Saturday night.

It was said Oakland's 36-year-old right-hander, who was making his first career start in the major leagues, threw no harder than 86 mph.

Don't believe it. Smith's fastball reached 88 mph against the Seattle Mariners at Safeco Field.

"I saw a couple snowmen," Smith said, referring to the two 8s. "I hope my wife got a picture of it."

Smith's first start was a quality one -- six innings with three earned runs allowed -- but he didn't factor into the decision as the A's scored in the ninth to win 4-3, getting the go-ahead run on Ryon Healy's two-out, ground-rule RBI double.

"That was goal, six innings with three runs," Smith said. "To do that in the big leagues ... that's a personal pat on the back."

Rajai Davis led off the ninth with an infield single against Seattle closer Edwin Diaz and stole second. With two outs, All-Star Yonder Alonso was walked intentionally.

Diaz got ahead in the count 0-2 to Healy, but Diaz then hung a slider that Healy hit into the gap in right-center field. The ball bounced over the wall for a ground-rule double, scoring the go-ahead run.

"Oh-and-two count, he made a mistake. Left it right in the middle of the plate," Mariners manager Scott Servais said. "In this league to middle-lineup guys, they're going to make you pay.

"Sometimes a tie game in the ninth inning comes down to one pitch."

A's closer Santiago Casilla allowed a two-out double in the bottom of the ninth to Mitch Haniger but got Jarrod Dyson to fly out to left field to end the game and secure his 15th save of the season.

The Oakland bullpen pitched three scoreless innings in relief of Smith, with Sean Doolittle (1-0) getting the victory.

Smith, who had made 63 relief appearances since breaking in with the Boston Red Sox in 2008, allowed six hits, walked one and struck out four. He left with the score tied 3-3.

"I thought he pitched great," A's manager Bob Melvin said. "I mean, that's not the easiest of lineups. ... He did every bit of his job and probably a little more. It was fun to see.

"He was bouncing around here a little bit, but there had to have been some nerves and a lot of excitement, and which was more I'm not 100 percent sure, but once got on the mound he did his thing like he did in Triple-A this year."

Smith held Robinson Cano and Nelson Cruz, American League All-Stars and the Nos. 3-4 hitters in the Seattle lineup, hitless in six at-bats.

"I paid attention to those guys. They're on ESPN and in the All-Star Game," Smith said. "I made pitches to them that looked like strikes but ended up balls."

What pitches were working best for Smith?

"I was throwing a fastball down the middle and when I did, they didn't hurt me ... except Dyson," Smith said.

Dyson's solo homer leading off the fifth inning tied the score at 3-3 and prevented Smith from perhaps earning the victory.

He didn't seem to mind.

"I got to enjoy it," he said. "I might be one-and-done."

Mariners rookie right-hander Andrew Moore, making his third career start, matched Smith. Moore also allowed three runs and six hits in six innings, though he walked two and struck out two.

The Athletics took the lead in the second as Healy led off with a single, moved to second an out later on a Bruce Maxwell single and scored on a two-out single by Jaycob Brugman.

Oakland extended the lead to 2-0 in the third on Alonso's solo home run with two outs. Alonso hit a 1-1 pitch from Moore 410 feet into the right-field seats for his 20th home run of the season.

Yet pretty much all Alonso wanted talk about after the game was Smith.

"He did a good job being himself," Alonso said. "You're looking at a guy having so much fun and enjoying every moment. He pitched quick and threw strikes and got ground balls, too."

The Mariners tied the score in the bottom of the third when Dyson led off with a double and scored on Jean Segura's one-out double. Ben Gamel followed with a run-scoring single to center to make it 2-2.

Marcus Semien gave the A's a 3-2 lead in the fifth on a solo homer with one out, his first home run of the season. Semien hit a 2-1 pitch from Moore that just eluded the reach of Gamel in left and bounced off top of the fence and over the wall.

Dyson tied it by leading off the bottom of the inning with his fifth homer of the season.

NOTES: The Mariners dropped to 1-9 in their past 10 games at home. ... The A's made a series of roster moves. They selected RHP Chris Smith's contract from Triple-A Nashville; reinstated C Josh Phegley from the paternity list; optioned INF Franklin Barreto, the organization's top prospect, to Nashville; and designated C Ryan Lavarnway for assignment. ... Oakland rookie RHP Daniel Gossett (1-3, 6.23 ERA) is scheduled to start the series finale Sunday against Mariners RHP Felix Hernandez (3-3, 5.04).