Named the American League's Pitcher of the Month earlier in the day, Haren celebrated with 7 2/3 innings of four-hit work and left with a one-run lead provided by Ellis, who hit a two-run triple in the second inning and a solo homer in the fourth.
Jason Kendall padded the lead with a sacrifice fly in the eighth, but the visiting Red Sox tied the game with two runs in the top of the ninth off interim Oakland closer Alan Embree, then turned back a bases-loaded, nobody-out threat in the bottom of the frame when Eric Chavez struck out and Bobby Crosby hit into a double play.
Given a shot at redemption two innings later, Chavez belted a solo homer to right off Boston righty Kyle Snyder to give the A's a 5-4 victory in 11 innings in the opener of a three-game series.
"I kind of felt like I threw away that one [in the ninth]," Chavez said. "It was nice to get another chance."
"I left the ball up, right over the plate," Snyder said of his fateful fastball. "He's a good hitter, and he'll take advantage of balls like that."
For much of the season, however, Chavez simply hasn't been that good. He entered Monday's game batting .150 (6-for-40) over his past 11 games, .233 overall, and he missed three games recently with what A's manager Bob Geren called triceps tendinitis.
Last week, Chavez cryptically told reporters that he'd have some sort of definitive medical news -- "everything you need to know in a couple of days," he said -- but he's been mum on the subject ever since and remained so after his breakout night.
"I'm just feeling better," he said after going 2-for-5 with a walk. "It was a good couple of days."
Whatever happened, the A's are thrilled to see Chavez swinging the bat with authority.
"They worked on him and got him healthy," Geren said, offering no further details. "I'm real happy for him. He's feeling good, and I think he's going to have a big second half for us."
Ellis, who added a double in the sixth and completed his cycle with a broken-bat single in the 10th, certainly hopes so.
"Chavvy means a lot to this team," Ellis said. "He means everything to this team."
As does Haren, who gave up a solo homer to David Ortiz in the first inning, a solo homer to Wily Mo Pena in the seventh and little else. After J.D. Drew's leadoff single in the second, Haren retired the next 14 Red Sox hitters in a row and 17 of 18 before Pena's homer.
"He's a dominant pitcher," said Boston manager Terry Francona. "He has velocity, good movement, and one of the better splits in the game. He's good. One of the best."
To which Ellis essentially said: One of the best?
"If there's a better pitcher in the American League," Ellis said, "I haven't seen him yet."
Eight of Haren's season-high-tying nine strikeouts came against the final 14 batters he faced.
"Another gutsy performance by Dan Haren," Geren gushed. "He was getting a little tired there in the seventh and eighth, but he reached back for a little something extra."
Lefty Jay Marshall got the final out of the top of the eighth, but Ortiz doubled to right to open the ninth and scored on a two-out single by pinch-hitter Jason Varitek. Coco Crisp came on as a pinch-runner for Varitek and scored from first on a long single to right-center by Pena.
After the A's blew their shot in the bottom of the frame, Ortiz struck again, drilling a two-out double high off the wall in center with Dustin Pedroia on first base. Center fielder Mark Kotsay, however, quickly tracked the ball down and got it to shortstop Crosby, who fired a strike to Kendall at the plate from medium center field. Pedroia was out standing up.
"I was out there, too, and by the time I turned around, the ball was already in Jason's glove," Ellis said. "That's not an easy play, but Mark got that ball off the wall real quick, and Bobby's got an amazing arm."
Mychael Urban is a national writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.