OAKLAND -- Marcus Semien knew center field was wide open. He just had to put the ball there.
Semien did it, beating a five-man infield with one out in the 11th inning to lift the Oakland Athletics over the Los Angeles Angels 6-5 on opening day Thursday.
"There was a lot of infielders and two outfielders, so I just tried to use the big part of the field there," Semien said.
Khris Davis hit a three-run homer in the fifth and a tying single in the seventh to almost single-handedly get the A's back in it.
Albert Pujols homered leading off the sixth, while Shohei Ohtani connected on the first pitch he saw for a single in a much-hyped major league debut for the Japanese two-way star.
Kole Calhoun and Cozart also homered for the Angels to back Garrett Richards, who began this season healthy following two injury-shortened years. Richards left his initial 2017 outing on April 5 at Oakland, then didn't pitch again for the Angels until Sept. 5 because of a rare nerve irritation in his biceps. He missed most of 2016 with a torn ulnar collateral ligament.
"I felt fine. The ball was coming out great. The stuff was good," Richards said. "I made one mistake to Khris Davis and unfortunately that happened to be the difference-maker today."
Batting eighth as the designated hitter, Ohtani grounded a hard single to right field off Kendall Graveman in the second inning. He grounded out his next three times up before a strikeout in the 11th.
"That's probably an at-bat I'm not going to forget for the rest of my life," Ohtani said through interpreter Ippei Mizuhara. "It's my first major league at-bat."
Ohtani often looked overmatched in the Cactus League, managing just four singles in 32 at-bats (.125) and striking out 10 times.
The 23-year-old Ohtani said he wasn't nervous as a hitter but "probably" will be when he starts on the mound for the first time in Sunday's series finale against the A's.
Davis began his quest for a third straight 40-homer season, and Matt Olson tied the game at 4 with a solo shot moments later.
Then Pujols chased Graveman with a drive to left - No. 615 of his career - the slugger's fifth career homer on opening day.
Graveman allowed five runs and seven hits in five innings with a strikeout and no walks in his second straight opening day start for Oakland. The A's played a day game home opener for the first time since 1994.
The Angels, who opened at Oakland for a second straight year, lost their fifth straight season opener after winning the previous five.
Moments before Semien's single in the 11th, plate umpire Ted Barrett approached Mike Scioscia during the Angels' sixth mound visit of the game and patted the manager on the back, apparently reminding him of his total. Teams are only allowed six mound visits per game beginning this season, excluding pitching changes, though teams get an extra visit for each extra inning. The rule change is part of baseball's initiatives to help speed up games.
The A's drew an opening day crowd of 27,764, nearly 21,000 shy of a sellout (48,592) in the newly configured Coliseum.
Angels: LHP Andrew Heaney, placed on the disabled list retroactive to Monday with inflammation in his pitching elbow, was scheduled to throw a bullpen Friday. "He felt really good coming out of his last bullpen," Scioscia said. "Just see how he feels after tomorrow's pen, during tomorrow's pen and after and maybe get a little more clarity." Heaney will pitch in a game somewhere before being activated, Scioscia said.