SEATTLE — Two pieces of what many consider to be one of the deepest bullpens in the game let down the Yankees on Saturday night.

After the offense completed a comeback from the early three-run deficit Masahiro Tanaka put the Yankees in, David Robertson gave up a leadoff homer to Robinson Cano in the bottom half of the eighth that gave Seattle the lead.

The Yankees rallied to tie it in the top of the ninth but Adam Warren allowed an RBI walk-off single to Nelson Cruz in the bottom of the 10th, a 6-5 loss to the Mariners in front of a sellout crowd of 46,197 at Safeco Field.

“The offense has been doing that all year, coming from behind,” Warren said. “It’s our job to hold the other team there until we can finish the comeback. Just didn’t have it tonight.”

To blame the bullpen for the defeat would be grossly unfair.

Tanaka offered some hope that he was getting back on track but in a bottom-line business, the bottom line is the righthander, the staff ace in title only, got a no-decision and is 7-9 with a 5.37 ERA.

He allowed four runs and seven hits in six innings, with all four runs, and five of the hits, coming in a four-run third that turned a 1-0 lead into a 4-1 deficit.

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The big blows in the inning were a leadoff homer by Mike Zunino and a one-out blast by Ben Gamel. Those made it 26 homers allowed in 20 starts this season for Tanaka, who allowed 22 all of last season.

“I wasn’t executing pitches, obviously, that inning,” Tanaka said through his translator. “But I felt like throughout the game, I really didn’t have my best stuff. I don’t think many of my pitches were sharp.”

Joe Girardi had a slightly different take on the outing, which started with Tanaka striking out three over the first two innings and, after the third, shutting the Mariners down and retiring the last 10 he faced.

“He had it, he lost it and then he found it again,” Girardi said.

The Yankees (50-46) fell to 5-5 on this 11-game trip that ends Sunday afternoon against the Mariners (49-50). The Bombers, who Sunday will try to win their first series since June 9-11 against Baltimore, are 9-19 in one-run games.

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Gamel, a former Yankee, led off the 10th with a double on a slider. An intentional walk to Cano followed and Cruz ripped a 2-and-1 slider to left to win it.

The Yankees had rallied against Mariners closer Edwin Diaz in the ninth when Didi Gregorius worked a leadoff walk and, with two outs, pinch runner Jacoby Ellsbury stole second. Ronald Torreyes then lined a 1-and-2, 97-mph fastball to left to tie it at 5.

The game included Aaron Judge’s MLB-leading 32nd homer of the season, an opposite-field shot in the sixth that cut the Yankees’ deficit to 4-3.

Former Yankee David Phelps, making his first appearance since the Mariners acquired him from the Marlins, came on in the seventh and retired four straight. But Mariners manager Scott Servais went with lefty Mark Rzepczynski to face Brett Gardner and the centerfielder singled. In came righthander Nick Vincent for rookie Clint Frazier, whom Girardi before the game called “the hot hand,” one he planned to play ahead of Ellsbury in the near future.

The rookie jumped on a first-pitch slider and doubled off the leftfield wall. After an intentional walk to Judge, Matt Holliday’s sacrifice fly to right tied it at 4.

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Robertson, acquired earlier in the week from the White Sox along with Todd Frazier and Tommy Kahnle, threw a first-pitch fastball to Cano, a former teammate in the Bronx, who lined it opposite-field to left, his 19th homer, giving the Mariners a 5-4 lead.

“That pitch was about as good as I could throw it,” Robertson said. “I don’t think I could do anything better with it. Just have to tip your hat to a really good hitter. Put a good swing on a good pitch.”