Martinez, who had an 11.08 ERA in his last three starts, was much better on Sunday against Iwakuma. But Seager, on a 4-for-4 afternoon, put the Mariners ahead with a two-run double in the fifth and they went on to a 5-1 victory over the Rangers.
The loss snapped the Rangers' three-game winning streak although they still took two of three from the Mariners to open their nine-game road trip. The Rangers' next stop is Oakland for a three-game series with the division-leading Athletics.
Seager also had a double in the Mariners' three-run eighth inning against relievers Robbie Ross Jr. and Ben Rowen, who was making his Major League debut.
"He doesn't miss much when we face him," Rangers manager Ron Washington said. "The first couple of days I thought we did a good job against him, but today when we made a mistake he didn't miss it."
In his last 42 games against the Rangers, Seager is hitting .392 with nine home runs, 31 RBIs, a .432 on-base percentage and a .667 slugging percentage.
"It's just one of those things," Seager said. "It's just sometimes the way it lines up, I guess."
Iwakuma has also had pretty good success against the Rangers lately and the only run scored off him on Sunday was a two-out home run by Brad Snyder in the second inning. It was Snyder's first Major League home run. At age 32, he is the oldest Ranger to hit his first home run since Hector Ortiz in 2002.
"I had never faced him before but his numbers speak for themselves," Snyder said. "It was definitely a get-it-over fastball and I was ready for it. I ain't going to lie. I've been waiting for the first big league home run for a long time. It's a big sigh of relief to get the first one out of the way."
The Rangers needed more than that but Iwakuma held them to six hits, did not walk a batter and struck out six. Snyder's home run was the only extra-base hit and the Rangers didn't get a runner into scoring position until two out in the eighth inning.
"The guy pitched his tail off," Washington said. "He can pitch. He kept the ball off the fat part of the plate and bore his sinker inside on our right-handed hitters. He was good."
Martinez, regaining command of all his pitches, went six innings and allowed two runs on nine hits. He walked one, struck out three and threw 96 pitches while giving the Rangers a fourth straight quality start. But unlike Yu Darvish, Nick Tepesch and Joe Saunders, Martinez ended up on the losing end.
"I felt good. ... I felt like I was back on track," Martinez said. "I had command of all of my pitches. It's just disappointing we didn't get the win, but it was definitely a step in the right direction."
Martinez was in trouble in almost every inning but was able to hold the Mariners scoreless through four. But when Endy Chavez led off the fifth with a single, it marked the fourth time in five innings that the Mariners' leadoff hitter reached on a hit. This time the Mariners didn't strand him.
James Jones popped out, Robinson Cano walked and Logan Morrison popped out. That brought up Seager, who had two leadoff singles in his previous two at-bats. This time he was batting with runners in scoring position and he doubled into the right-field corner to give the Mariners a 2-1 lead.
"Game management," Martinez said. "I probably should have pitched around him or made him chase something outside of the zone."
Martinez got out of the inning by getting Mike Zunino to fly out, then he retired the side in order in the sixth for the first time on the afternoon.
"He found his secondary stuff today," Washington said. "Threw a lot of changeups and threw some breaking balls. With Nick, you need all of your pitches to maneuver through the batting order and he did. He made the mistake to Seager but it was certainly a big improvement."