"I thought I had good enough stuff to get people out, but I just kept leaving pitches up, and those guys are too good to leave pitches in the zone," said Robertson, who coughed up two runs in the ninth, including Kelly Johnson's walk-off RBI double.
Pitching for the third straight day and having acknowledged soreness in his back, sides and hamstrings after firing 46 pitches earlier in the weekend, Robertson also surrendered doubles to Nelson Cruz and Steve Pearce, with Pearce's tying the game.
Johnson, who opened the season as the Yankees' Opening Day third baseman, was mobbed at second base in a pile of his Baltimore teammates after his loud drive up the gap in right-center field; perhaps the final nail into New York's postseason aspirations.
"It's really, really exciting," Johnson said. "I'm not going to lie. It feels really good. You want to contribute, you want to drive in runs, you want to score runs, all those things and you want to be a part of it."
New York is now five games back in the Wild Card chase with 14 games to play. Girardi defended his decision to use Robertson, who blew his fourth save in 40 opportunities this season.
"I mean, he's my closer. That's the thing," Girardi said. "It's the time of year. That's why I try to take care of him all year long. You get to September and sometimes you've got to do that. Like I said, he's been great for us all year, and it just didn't work out."
Brian McCann gave the Yankees the lead in the top of the ninth with a homer to right field off Darren O'Day. McCann has now reached the 20-homer plateau in seven consecutive seasons, having accomplished the feat in each of the last six years with the Braves.
Dellin Betances recorded two strikeouts in a scoreless eighth inning, giving him 130 for the season and tying Mariano Rivera's single-season franchise record for a reliever, set in 1996.
"It's a great accomplishment, especially after everything I've gone through to get up here," Betances said. "I'm honored to be a part of this team, and for me to just be in the same area, or just by Mariano, that's a huge accomplishment for me. He did it with one pitch though -- that's the big difference."
Girardi said that he did "absolutely not" give consideration to pitching Betances for the ninth inning, saying that "Dellin has been used a lot, too."
Hiroki Kuroda continued what has been a strong finish to the season, limiting the Orioles to a run on six hits over seven innings. Kuroda walked none and struck out five in a 94-pitch outing, throwing 66 for strikes.
Baltimore's only dent against the right-hander came in the sixth. Alejandro De Aza opened the inning with a single and scored on Adam Jones' line drive double to left, which tipped the glove of leaping third baseman Martin Prado to stay fair.
"Dellin and Hiro did a great job, and I wish I could have nailed it down," Robertson said. "I didn't have good enough stuff tonight."
With the start, Kuroda passed the 3,000 innings mark for his professional career, having pitched 1,700 1/3 innings for the Hiroshima Toyo Carp from 1997-07 and entering Sunday's game with 1,297 1/3 innings in the Majors with the Dodgers and Yankees.
Prado's second-inning home run, his 12th of the season, accounted for the only run that the Yankees produced against right-hander Chris Tillman, who scattered five hits over 6 2/3 innings.
"It's very difficult because of what we're trying to do," Girardi said. "Each game that you lose like this, it just makes it harder and harder to get to where we want to get."
Retiring Yankees captain Derek Jeter, who was honored in a pregame ceremony that included Orioles fan favorite Boog Powell, manager Buck Showalter and outfielder Nick Markakis, went 0-for-4 with a strikeout and is hitless in his last 24 at-bats.
"I've always talked about [how] you want to be in a position where you control what happens," Jeter said. "Unfortunately, now we're not in control of what happens. We're in control of our games, but now you need help from other people. We're at the point now where playing well isn't good enough."