Despite the loss Boston's lead over Tampa Bay for the AL wild-card spot increased to 2 1/2 games. The Rays were swept by the New York Yankees in a day-night doubleheader.
Ace Josh Beckett took the mound, hoping to put some distance between Boston and the Rays. Beckett (13-6) allowed just one hit through five innings as the Red Sox built a 4-1 lead.
But Beckett allowed a run in the sixth and two more in the seventh on Mark Reynolds' second homer that tied the game.
In the eighth, Beckett gave up a single to J.J. Hardy and a double by Nick Markakis. Alfredo Aceves relieved with runners at second and third, then gave up Guerrero's hard, two-run single to center field.
One night before Boston's best starter faltered, star closer Jonathan Papelbon blew a save chance for only the second time this season when he gave up a three-run double to Robert Andino in the eighth that gave Baltimore a 7-5 win.
The Boston collapse has come after it led Tampa Bay by nine games on Sept. 3.
The Red Sox have six games left, all on the road — three at Yankee Stadium and three at Baltimore. Tampa Bay has seven remaining — four against the Yankees and three against Toronto.
And the Los Angeles Angels are sneaking up. They're just 2 1/2 games behind Boston after beating Toronto 7-2 on Wednesday.
Clay Rapada (2-0) got the win and Jim Johnson earned his ninth save in 14 chances – his third in as many days against Boston. He pitched a perfect ninth and, after Jed Lowrie made the final out on a soft grounder to Johnson, the fans booed.
Carl Crawford had a single, double and triple for Boston and drove in two runs.
Reynolds hit a solo homer in the second before Boston tied the game in the third on a triple by Crawford and an RBI groundout by Jason Varitek.
Crawford gave the Red Sox a 3-1 lead in the fourth with a two-run double after a single by Adrian Gonzalez and a double by Dustin Pedroia. Consecutive singles by Mike Aviles, Gonzalez and David Ortiz made it 4-1 in the fifth against Tommy Hunter, who left the game with two outs in the seventh with a strained right groin.
The Orioles added a run in the sixth when Andino singled, stole second and scored on a single by Hardy. They tied it in the seventh when Matt Wieters singled and was forced out at second on a grounder by Adam Jones before Reynolds hit his 36th homer of the season — his second of the game over the Green Monster in left field.
Notes
Reynolds tied a career-high with two homers in a game. It was the fifth time he did it this season and the 12th time in his career. … After a day off on Thursday, the Red Sox begin a season-ending six-game road trip with Jon Lester (15-8) facing the New York Yankees in the opener of a three-game series on Friday night. Boston finishes the season with a three-game series in Baltimore starting Monday. … With two hits, Gonzalez passed Mo Vaughn for the most hits in a season by a Red Sox first baseman. Gonzalez has 208, one more than Vaughn had in 1996. … Jacoby Ellsbury got a hit for his 33rd consecutive game against Baltimore. It's the longest hitting streak against the Orioles franchise and the longest by a Red Sox player against any team.