SEATTLE -- The Oakland A's knew better than most that a 5-0 lead in the second inning wasn't reason to relax.

"My mindset was to tack on more runs," shortstop Marcus Semien said after hitting a two-run homer as the A's jumped on top of the Seattle Mariners 5-0 Tuesday. "Unfortunately, it didn't happen."

One night after coming back from a 5-0 deficit, the A's were on the other side of that equation in a 6-5 loss to the Mariners. Seattle rallied to score six times in the fourth and fifth to earn the comeback win on this night.

"It didn't look good early," Seattle manager Lloyd McClendon said.

Home runs by designated hitter Nelson Cruz, hit 38th of the season, and first baseman Logan Morrison sparked the turnaround after Seattle starter Mike Montgomery struggled through the shortest outing of his young career.

Montgomery lasted just 1 1/3 innings, allowing five runs on seven hits, but he did not factor into the decision. It marked the first time in his past nine starts that Seattle (58-68) won the game.

Mariners left-hander Edgar Olmos (1-0) earned his first career win after pitching 3 2/3 innings of scoreless relief. Olmos came on for Montgomery in the second inning and allowed just three hits while pitching through the fifth.

Tom Wilhelmsen entered in the ninth and gave up a two-out double to pinch hitter Sam Fuld before Oakland right fielder Josh Reddick flied out to right to end the game.

Oakland starter Jesse Chavez (7-13) allowed six runs on eight hits over 4 2/3 innings.

"After a good start, and a big lead, he just couldn't hold them down," manager Bob Melvin said of Chavez.

The A's right-hander was most upset by a fifth-inning infield single from Mariners second baseman Robinson Cano that kept the inning alive after Chavez tried to make a stab at the hard-hit grounder.

"I've got to let that ball go," Chavez said. "That's all on me today. I should've gotten out of that inning."

Chavez walked the next batter before Seattle center fielder Austin Jackson delivered the go-ahead hit with a bases-loaded single, driving in a pair of runs.

Center fielder Billy Burns went 3-for-5 and scored twice for the A's (55-72). Semien hit his 11th home run of the season, and his third in four games at Safeco Field this year.

Seattle rookie shortstop Ketel Marte went 3-for-5, finishing a home run short of the cycle. He doubled in the first inning, hit his first career triple in the sixth and added a bloop single in the eighth.

Five of the Mariners' six runs came with two outs.

The A's, who outscored Seattle 11-0 over the final five innings Monday, picked up right where they left off while jumping out to a 5-0 lead Tuesday. A two-run homer from Semien highlighted a four-run second inning.

After catcher Josh Phegley gave the A's a 1-0 lead with an RBI single in the first, Oakland opened the second inning with five hits to put an early end to Montgomery's night. Semien's blast, an RBI double from right fielder Josh Reddick and third baseman Danny Valencia's sacrifice fly gave Oakland a 5-0 advantage.

Seattle got back in the game with the three-run fourth. Cruz provided the Mariners' first run of the game with a leadoff shot. After Jackson contributed a two-out double off the wall, Morrison took Chavez deep for a two-run shot off the right field foul pole, pulling Seattle within 5-3.

In the fifth, Cano's infield single cut Oakland's lead to 5-4 before Jackson put the Mariners in front.

Seattle's bullpen -- Olmos, Logan Kensing, Carson Smith and Wilhelmsen -- threw 7 2/3 scoreless innings to finish off the win.

"Just a tremendous job," McClendon said. "They really saved us today."

NOTES: Nelson Cruz was in the lineup as Seattle's designated hitter after taking a pitch off his right elbow the previous night. Cruz was able to finish Monday's game and apparently came out of it with nothing more than a bruise. ... The game was paused for a minute or so in the seventh inning after Cano recorded his 30th double of the season. He became the first player in major league history to record 30 doubles in 11 consecutive seasons to begin a career. ... The Athletics plan to re-evaluate LHP Felix Doubront's injured right foot Wednesday. He took a line drive off the arch of his foot in Monday's win at Seattle. ... Athletics 2B Brett Lawrie played despite being visibly shaken by a pregame incident involving his grandmother near the visiting dugout. About an hour before the game, she passed out on the field and had to be taken off on a stretcher. Lawrie, who broke down on the field and had to be consoled by his sister and fiancee, did not want to talk about the episode after the game.