On the night Giambi hit his 400th career home run and should have been celebrating, he was left to ponder another one-run defeat.
"Right now it really doesn't mean that much," Giambi said. "Individual accomplishments are nice, but I'd rather have some wins and get the team on track. We've got to stop giving away games. We've got to get on the same page. ... Hopefully we can turn it around before we bury ourselves."
Giambi became the 44th player in major league history to reach 400 homers. Jack Cust, Adam Kennedy and Nomar Garciaparra also connected as the A's pounded former ace Dan Haren, but still couldn't pull out the win.
Mark Reynolds added a two-run double in the 11th off Santiago Casilla, Oakland's seventh pitcher, and Tony Pena (4-1) pitched the 10th for the victory. Arizona rallied against Craig Breslow (0-1) and improved to 6-2 during its three-city road trip that winds up Sunday.
"Come-from-behind victories are always fun. This road trip we've gotten our swagger back and our determination that we're never out of it," D-backs manager A.J. Hinch said.
Chad Qualls finished for his 11th save in 12 chances, but not before giving up Kennedy's run-scoring double followed by Ryan Sweeney's infield RBI single. Jack Hannahan then grounded into a game-ending double play with the tying run on third.
"It's not any secret what you want to do in the situation," Qualls said. "You take a look and you realize you can't let the guy score. You think 'How will I get out of this?' I took a deep breath and went to the sinker."
Giambi connected for the milestone drive leading off the fourth inning, hitting a 3-2 off-speed pitch into the right-field bleachers and watching the ball as it sailed out. Haren's four homers were a career high and doubled his previous season total.
The 38-year-old Giambi received a warm ovation from the crowd of 21,295 and then came back out of the dugout for a curtain call at the Coliseum. The solo shot put the A's up 3-1, then Kennedy homered two batters later. Garciaparra's sixth-inning solo shot made it 5-1.
That instant offense helped stake Edgar Gonzalez to a nice cushion in his Oakland debut while facing his former club, but the bullpen couldn't hold a four-run lead. Gonzalez made 35 starts for Arizona from 2003-2008, then signed a minor league deal with the A's in February. He had seven starts for Triple-A Sacramento before his callup Tuesday.
"I hoped to fight for the win, especially because it was the Diamondbacks," Gonzalez said.
Giambi's fourth homer of the year also marked his 191st with the A's, tying Gus Zernial for ninth place in franchise history. Giambi returned to Oakland this year for a second stint following seven seasons with the Yankees.
The A's drafted Giambi in 1992 and he made his major league debut three years later.
"It's an incredible thing," Giambi said. "I've had lots of ups and downs. The biggest thing is it was here in Oakland. I wish it could have been sooner, but I'm glad they got to see it, where I started my career."
Oakland lost its third straight and seventh in nine to fall a season-high 10 games under .500 at 15-25.
After Drew hit a sacrifice fly in the top of the first, Cust answered in the bottom half with a two-run homer.
Haren is winless in his last three starts. Traded to Arizona after the 2007 season, he dominated the A's when they met in interleague play last year.
Arizona loaded the bases with no outs in the eighth to chase Russ Springer, then former A's fan favorite Eric Byrnes hit a two-run single one out later off Andrew Bailey before Chad Tracy's tying two-run double.
Arizona leadoff man Felipe Lopez left with tightness in his left hamstring following his fifth-inning single and was listed as day-to-day. He doubled to start the game.
Arizona ace Brandon Webb, on the DL with tightness in his pitching shoulder, played catch and will continue to be pushed in his throwing program. "He will start to increase the volume and distance the next homestand," Hinch said. "Each day, he's getting a little bit stronger." ... The D-backs were 1-5 on the road in interleague last year.