One pitch later, Marte homered to right-center.
"Bad start, bad start. All nine hitters, the ball was up in the zone and down the middle of the plate," Saunders said. "I could go on and on. Just a bad one."
After Andrew McCutchen's two-out double, Blake Beavan replaced Saunders, who is 10-3 in 17 starts at Safeco Field. His ERA at the park rose from 2.31 to 2.80.
"I think it was a lack of execution, and he'll be the first to tell you," Hurdle said. "We saw his 1-0 complete game. We've seen him effective many times. Making the pitch he needed to or wanted wasn't really there. It happens up here."
The Mariners scored a run in the second on Carlos Triunfel's two-out single to center. Seager opened the sixth with his home run to right on a 2-1 pitch from Locke.
Marte hit his eighth homer with two outs in the eighth on the first pitch from Carter Capps. Sanchez hit his seventh in the ninth off Capps.
NOTES: The last time Saunders worked just 1 2-3 innings was Aug. 7, 2009, with the Angels against the Rangers. His career low is two-thirds of an inning on Sept. 30, 2006, with the Angels against the A's. ... The Pirates have had eight scheduled days off this season, including one Monday. They are 8-0 in the first game after an off day. ... Before the game, the Mariners placed CF Franklin Gutierrez on the 15-day disabled list (strained right hamstring) and recalled OF-2B Dustin Ackley from Triple-A Tacoma.