Ex-Ranger Justin Smoak hit what would be a game-winning home run in the eighth and added an RBI hit in the ninth to ruin his former team's comeback attempt.
Meanwhile, the A's won again, beating the Orioles, 5-2, to cut the Rangers' lead in the division to two games.
So why does the No. 5 spot matter so much? Because Texas plays seven of its final 17 games against Oakland, and the way things are lined up now, that fifth spot will be needed twice against the surging A's, the first time on Sept. 26 at home, the other in Oakland during the final three-game series of the regular season.
And although the other four members in the rotation are locked in and going very well, Feldman isn't.
Feldman, who had lost five straight decisions going into Saturday, earned a no-decision on this night. Still, he allowed six runs in 2 2/3 innings, and left the Rangers trailing, 6-0, when he departed.
Feldman's struggles leave an opening for someone to take his place, perhaps Martin Perez, who came in behind him and threw no-hit baseball for 4 1/3 innings.
Manager Ron Washington said after the game that he hadn't had time to evaluate the fifth spot in the rotation, and didn't have a comment about whether Feldman would get another start.
Washington did have a lot of accolades for Perez, who walked one and hit another of the 15 batters he faced. He struck out five.
"I thought he was great," Washington said. "He came in and kept the ball down. He had a good changeup. He was aggressive."
Perez, who has made three starts this season with a 2.60 ERA, said after the game that he can be ready to start if needed. He has made two strong appearances in a row -- the other against Tampa Bay for five innings.
"I can be ready if they give me the right amount of days," Perez said. "I'm here to help the team win."
The Rangers nearly overcame Feldman's poor start, rallying from a five-run deficit in the sixth and seventh innings. Mike Napoli, returning from a month-long stint on the disabled list, closed the gap with a three-run homer to left field with two outs in the sixth inning to make it 6-4.
"We all know he's capable of swinging the bat," Washington said.
The Rangers manufactured two runs to tie the score in the bottom of the seventh. Josh Hamilton had a one-out walk and Adrian Beltre followed with a single. Nelson Cruz sneaked a single under the glove of Mariners third baseman Kyle Seager to cut the lead to one run. After an intentional walk to Michael Young, David Murphy hit a slow roller to the right side to score Beltre for a 6-6 tie.
Seattle regained the lead immediately, as Smoak, batting .190 entering the game, smoked a line drive into the center-field bleachers in the top of the eighth for a one-run lead. The Rangers were forced to use Tanner Scheppers in a tie game in the eighth with Mike Adams unavailable until Sunday because of a tight trapezius muscle.
"He just got the ball up, and Smoak hit it," Washington said. "I wish he had pitched like he did after that to Smoak."
The Rangers had a chance in the bottom of the ninth, as Young reached on an error and Murphy on an infield hit. But Napoli skied a fly ball to right, and pinch-hitter Mitch Moreland was struck out by Mariners closer Tom Wilhelmsen.
Feldman allowed a run in the first but pitched a perfect second, getting two strikeouts. Then the hits started coming. Dustin Ackley homered to start the third inning, the first of five hits the Mariners would get en route to scoring five runs and taking a six-run lead.
It may have been that one inning that will force a change in No. 5 starters.
"I can't worry about that," Feldman said. "I'm only concerned with what I control."