Semien hit a grand slam off George Kontos in the sixth inning and Oakland held on to beat the San Francisco Giants 8-5 on Monday night, a few hours after shipping ace Sonny Gray to the New York Yankees in the second of two trades orchestrated by the A's front office.
Jed Lowrie had three hits and an RBI, Ryon Healy added a two-run single and Matt Joyce reached base four times and scored twice to help the A's get the upper hand in the Bay Bridge Series before a crowd of 38,391.
"I think really the distraction of the day was mitigated by the crowd and playing against these guys," Oakland manager Bob Melvin said. "We don't get too many crowds like that on a Monday night so when you do lose a couple guys like Sonny and (Adam Rosales), to be able to go out and have that kind of atmosphere kind of gets you past that."
Bruce Maxwell drew a leadoff walk from Josh Osich (3-2) to begin the sixth. Pinch-hitter Rajai Davis followed with a one-out single before Osich walked Joyce to load the bases. Kontos replaced Osich to face Semien, who powered a 1-1 pitch over the wall in left-center for his second career grand slam.
"I was just trying to get a good pitch to hit," Semien said of his fourth home run this season. "I knew Kontos liked his slider so I chased one first and then I just regrouped and tried to see the ball up."
Three years after winning its third World Series in a five-year span, San Francisco fell to 40-67, the worst record in baseball. The Giants dropped 35 games behind the Dodgers in the NL West -- the most they've been behind since the final day of the 1946 season.
Manager Bruce Bochy pointed out the nine walks issued by his pitchers, six from the bullpen. Four of the A's who walked scored.
"In the major leagues that shouldn't happen and that's what did us in," Bochy said. "Of course the grand slam, that made it a steep uphill climb for us. But we have to control the ball better. We have to get better in the bullpen. That's really hurt us this past week."
Earlier in the day the A's shook up their roster with a pair of trades. Gray was shipped to the Yankees in exchange for three minor leaguers, and infielder Rosales was sent to Arizona for minor league pitcher Jeferson Mejia.
CHAMPIONSHIP FIRST PITCH
Klay Thompson of the NBA champion Golden State Warriors threw out the ceremonial first pitch . Thompson, whose brother Trayce plays for the Los Angeles Dodgers, threw a sharp strike to Semien, who was behind the plate.
Giants: RHP Johnny Cueto was shut down after three scoreless innings of his rehab start with Class-A San Jose because of forearm tightness. Cueto (blisters) had been scheduled to throw about 70 pitches but was removed after 34. ... Closer Mark Melancon (right pronator strain) threw 15 pitches in one scoreless inning of work for Triple-A Sacramento on Sunday. The team has yet to decide when Melancon will throw again.
Athletics: IF/OF Chad Pinder was reinstated from the 10-day disabled list after being out more than a month with a strained left hamstring.
Giants RHP Jeff Samardzija (5-11, 4.85 ERA) will attempt to win consecutive starts for only the second time this season, while the A's will counter with LHP Sean Manaea (8-5, 3.82 ERA) in the finale of the two-game series Tuesday at the Coliseum. Manaea has won seven of his last nine decisions.
Shohei Ohtani, the 23-year-old Japanese right-hander with a 100 mph fastball and devastating left-handed swing, met with the Mariners and Cubs on Tuesday and was meeting with the Rangers on Tuesday evening, according to multiple reports.
New Marlins CEO Derek Jeter said the team is "still gathering information" on potential deals for Giancarlo Stanton, adding that "anything up to this point has been speculation."
Leonys Martin, who hit .172 last season with the Mariners and the Cubs, agreed with the Tigers on a one-year, $1.75 million contract.
Wily Peralta will make $1.5 million guaranteed with the Royals in 2018.
The Angels signed infield prospect Kevin Maitan after the Braves lost him in a penalty for improper international signings.
Reliever Dan Otero, who is 8-1 with a 2.14 ERA in 114 appearances over the past two seasons, has re-signed with the Indians on a two-year deal.