Fister (3-5) picked up his first victory since April 30, a 4-3 decision over the Baltimore Orioles on Monday.
Fister went 7 1/3 innings, allowing three runs and seven hits, walked one and struck out a career-high nine in his 49th start. He threw 23 of 30 first-pitch strikes. He is 3-2 with a 3.02 ERA over his past eight starts, but this was his first win in May.
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"Guys have been gaining confidence. It's a great thing to be a part of," Fister said. "It's a real family atmosphere here."
The Mariners, who lost 11 of their first 15 games to open the season, have won 10 of their past 12 and are 19-11 over their last 30 games to go one game over .500 (27-26). They have assured just their second winning May (14-11) since 2004.
Fister worked at least into the seventh in four of his five May starts with a 3.79 ERA but had nothing to show for it.
"You look at his stuff, the way he approaches his game plan and sticks to it, the way he's able to make adjustments and make pitches," Mariners manager Eric Wedge said, "He's not just a one- or two-pitch guy. He has weapons beyond that. What I see a guy who really understands himself."
Brandon League worked the ninth for his 14th save in 17 opportunities.
Jack Cust supported Fister with his second career triple, driving in two runs.
Ichiro Suzuki, steeped in his worst career slump -- 1 of 22 over his previous five games -- had two singles and scored twice. He has a career .367 average against Baltimore, the best against any team. He has at least one hit in 40 of the last 42 games he has played against the Orioles.
The Orioles have lost four straight.
Jake Arrieta (6-3) lasted just 2 1/3 innings for the Orioles, allowing four hits, four runs and walking four.
Suzuki greeted Arrieta with a sharp first-inning single to right. With one out, Justin Smoak walked and Cust followed with a triple. The high flyball hit halfway up the right-center wall then bounced away from the fielders.
His first triple came on Aug. 18, 2007, for Oakland against Kansas City.
"There's just not many opportunities to get a triple," Cust said. "The ball kind of fell away from them so it was pretty easy."
Wedge added, "I told him you have to fill those columns and he filled one today."
Mark Reynolds drilled the first hit against Fister in the third, a solo shot into the second deck in left field. It was his seventh homer.
Suzuki then initiated the Mariners' two-run, third inning with a leadoff single that caromed off Arrieta's left leg. Brendan Ryan followed with a single to left, extending his hitting streak to a career-high 10 games. Smoak walked to load the bases.
Fister yielded a leadoff double to Luke Scott in the fifth but didn't allow him to advance past second. In the seventh, he had runners on first and second with one out then struck out Nolan Reimold and got Reynolds on a fielder's choice.
"He got ahead and it is always tough to hit when you are 0-1 or 0-2," Matt Wieters said of Fister. "Next time you have to come out and be aggressive so he doesn't get to 0-2."
The Orioles rallied for two runs off Fister in the eighth. Robert Andino opened with a one-hop shot that third baseman Chone Figgins couldn't handle. Felix Pie then singled to right with Andino racing to third. With one out, Nick Markakis singled to right, scoring Andino to make it 4-2.
Aaron Laffey ended the rally by getting Scott on a grounder.
League allowed Reimold's leadoff single in the ninth but finished the inning on a double play by Andino.
Earlier this month, League allowed the Orioles for rally for two last-at-bat victories against him.
"I don't really think about that anymore. I analyzed that and moved on," League said. "But now that you say that, it does feel good."
League is tied for the league lead in saves.
Figgins, struggling with a .193 batting average, was moved from second to eighth in the Mariners' lineup. "I've been through stuff like this before. My mind says I'm not going to change. I'll try to do the same things, play defense and try to create some havoc." Manager Eric Wedge says Figgins is looking for the "feel" a hitter needs. "That's what he's lost and what he's working to get back," he said. Figgins was 0 for 3 with two strikeouts and an error. ... RHP Chris Jakubauskas returned from rehab (strained groin) at Triple-A Norfolk. He replaced RHP Chris Tillman, optioned to Norfolk. ... Orioles CF Adam Jones sat out Monday as a precaution because of a sore shoulder. He is day to day. ... LHP Brian Matusz will come off his rehab assignment (strained intercoastal) and start Wednesday against Seattle.