Rivera's first wild pitch of the season -- and second in six years -- allowed the tiebreaking run to score in the ninth inning, and Jason Giambi flied out with the bases loaded to end the Kansas City Royals' 4-3 victory over the fading Yankees on Friday night.
"I've been a catcher, a broadcaster, a coach, a manager -- I can't remember him throwing a wild pitch," New York skipper Joe Girardi said. "You never see it."
Mark Teahen hit a two-run double and Kansas City got another solid outing from Gil Meche, who hasn't lost in eight starts since July 2 at Baltimore. The last-place Royals improved to 6-29 at Yankee Stadium this decade by beating New York for the fifth time in eight meetings overall this season.
With a 3-2 count on Johnny Damon, pinch-runner Justin Christian was inexplicably picked off first base for the first out of the bottom of the ninth, the latest abysmal play by an injury-depleted Yankees team that is falling fast in the playoff race.
"I was running on that pitch. You just can't get picked in a situation like that," Christian said. "I was too aggressive."
New York, which hasn't missed the postseason since 1993, dropped seven games behind Boston for the AL wild-card spot.
After Joakim Soria picked off Christian, the Yankees loaded the bases with two outs. But the All-Star closer preserved Kansas City's second victory in nine games when Giambi hit a routine fly to center.
"Soria stayed with it, which is tough to do in that situation, especially in this venue," Royals manager Trey Hillman said.
Esteban German doubled with one out in the top of the ninth and moved to third on Mitch Maier's infield single. With Mike Aviles up, Rivera bounced a pitch that skipped off catcher Jose Molina and far enough behind the plate for German to scurry home without a play.
"It was a sinker. I was trying to get a groundball. It just happened," Rivera said.
Leo Nunez (4-1) worked 1 2/3 scoreless innings for the win and Soria got three outs for his 33rd save in 36 chances.
Kansas City beat Rivera (4-5) on June 9 to split a four-game series at Yankee Stadium.
"Against him, you take anything you can get," Royals first baseman Ross Gload said.
During a 19-game stretch that has them playing 16 on the road, New York stopped home for a three-game set against Kansas City. Coming off a 3-7 trip, the Yankees demoted slumping center fielder Melky Cabrera to the minors and cut backup first baseman Richie Sexson as part of a mini shake-up.
Brett Gardner was recalled from Triple-A and given a chance to hold down the everyday job in center. A light-hitting speedster, he went 0-for-4 but threw out a runner at the plate.
New York's Andy Pettitte struck out his first two batters after the start of the game was delayed 91 minutes by rain, but Kansas City jumped ahead in the second inning.
John Buck snapped an 0-for-30 slide with a leadoff double in the fifth and the Royals loaded the bases with one out for Teahen. His ground-rule double made it 3-0 -- though German came tearing around third unnecessarily and nearly passed Buck steps from home plate.
Buck even held out his arm, signaling his teammate to slow down and not run into him.
The Yankees began their comeback with a leadoff double by Molina in the bottom half. They were 0-for-9 with runners in scoring position before Derek Jeter's RBI single.
"We didn't execute. Because we didn't execute, we didn't win," Girardi said. "At some point you have to start executing if you want to play in October."
Meche was lifted with a 3-1 lead after six innings and 106 pitches. He struck out eight and walked two.
Ron Mahay's wildness helped New York rally in the seventh. The left-hander walked Damon and Jeter with one out before Bobby Abreu's RBI single off Nunez cut it to 3-2. Alex Rodriguez tied it with a sacrifice fly.
Jeter returned to the lineup after missing Wednesday's loss at Minnesota with a bruised foot. ... Pettitte hasn't lost to the Royals since August 1999, though they scored 10 earned runs off him June 7. ... German ended an 0-for-17 skid with a leadoff double in third. He was thrown out at the plate by Gardner on Aviles' bloop single. ...Yankees designated hitter-left fielder Hideki Matsui hit a solo homer in his first minor league rehab game with Class-A Tampa on Friday. Matsui, on the 15-day disabled list since June 27 with an injured left knee, went 1-for-3 as the DH.