SAN FRANCISCO -- In the great debate of who got the better of the Marcus Semien-Jeff Samardzija trade in December 2014, the Oakland Athletics' import made the loudest statement Monday night.

Semien blasted a three-run homer off his former trade partner in a five-run second inning and rookie Daniel Mengden posted his first major-league win with 7 2/3 strong innings as the A's blitzed the San Francisco Giants 8-3 in the opener of a four-game interleague series.

Semien drove in four runs and Khris Davis two, helping the A's extend the all-time lead over their San Francisco Bay Area rival to 55-52.

The series continues Tuesday night in San Francisco, after which it moves across the San Francisco Bay Bridge to Oakland for games Wednesday and Thursday.

"I wouldn't be here if not for him," said Semien, whose cross-country flight path crossed with Samardzija's when they were the biggest names in a six-player deal between the A's and Chicago White Sox 18 months ago. "Every time I see him, I know he's the guy I was traded for."

Semien came to the plate after Davis doubled home Danny Valencia with the game's first run in the top of the second.

A walk to Yonder Alonso put two aboard for Semien, who smacked his 15th home run of the season over the fence in left-center field.

The home run allowed Semien to match his career-best total set last season. He ranks fourth among shortstops in homers this year, trailing just Trevor Story (19) of the Colorado Rockies, Manny Machado (18) of the Baltimore Orioles and Corey Seager (16) of the Los Angeles Dodgers.

"He just continues to put up numbers," A's manager Bob Melvin said about Semien. "He's a true two-way player. I don't know how his name doesn't come up in possible All-Stars."

The A's weren't done with Samardzija (8-5) in the second after Semien's homer. A single by Coco Crisp and RBI double by Jed Lowrie completed a five-run, five-hit inning.

The uprising assured that Samardzija, signed by the Giants as a free agent in December, would allow at least five runs for a fourth time in his last six starts.

"It's just about eliminating those mistakes that come back to haunt you," Samardzija said of his slump. "It's just a mental mistake when you're 0-2 there to Semien and you leave one over the plate. It's just about slowing the game down right now and making the pitch that needs to be made."

The five runs were plenty for Mengden (1-3), who had received just two runs of offensive support in his first three starts.

"It's good that we came with our hot bats," Semien said. "I'm impressed (with Mengden). He's come up here and shown he belongs."

Facing a first-place Giants team that had averaged 5.0 runs in its last 15 games, the right-hander retired the first 13 batters he faced and took a shutout into the eighth.

Mengden left two outs into the eighth, having allowed a run on a double-play grounder by Giants reliever Chris Stratton.

Right-hander Fernando Rodriguez's wild pitch allowed Conor Gillaspie to score a second Giants run in the eighth before he struck out Brandon Belt to shut the door at 8-2.

Mengden was charged with both of the runs. Pitching his first big-league night game, the 23-year-old allowed four hits and three walks in a season-best 7 2/3 innings, striking out five.

"It feels real good," Mengden said. "I just try to control what I can control. I can't do anything about the offensive side. I just do my best to keep us in it."

The Giants closed within 8-3 in the ninth on shortstop Ramiro Pena's second hit of the game, a two-out RBI single off Zach Neal.

Left-hander Marc Rzepczynski came on to retire Gillaspie on a grounder to short to end it.

Valencia had three singles, and Davis and Lowrie joined Semien in the two-hit club for the A's, who lost the season series 5-1 to the Giants last year.

The A's out-hit the Giants 12-7.

The win improved Oakland's record to 3-5 in interleague play this season. The A's have posted a winning record against the National League in each of the last four seasons.

The loss prevented the Giants from becoming the first 50-win team in the majors. It was just their fourth loss in their last 17 home games.

"He's been pitching well," Giants manager Bruce Bochy said of Mengden. "He came into this thing 0-3, but you look at his games, he's been throwing the ball very well. He's got more strikeouts than innings pitched, which is the sign that he's got good stuff."

The Giants' first hit off Mendgen came with one out in the fifth on a single by Angel Pagan. Mengden then walked Gregor Blanco to put two aboard before retiring Pena and Gillaspie on fly balls to retain the 5-0 lead.

Meanwhile, Samardzija rebounded from his rough second inning to retire the A's in order the next time through the lineup before back-to-back doubles by Alonso and Semien made it 6-0 in the sixth.

Samardzija was pulled for a pinch hitter in the bottom of the inning, having allowed six runs and eight hits in six innings. He walked two and struck out two.

NOTES: A's SS Marcus Semien's four RBIs tied a career high, set April 23, 2014, for the Chicago White Sox against the Detroit Tigers. ... The win was the just the A's fourth in their last 20 meetings with the Giants in San Francisco. ... A's RHP Daniel Mengden went 0-for-4 at the plate, making Oakland pitchers 29-for-362 (.080) all time in interleague play, worst in the majors. ... Giants RHP Jeff Samardzija has a 6.89 ERA in his last six starts. ... The A's hope to have RF Josh Reddick (broken left thumb) back from the disabled list for the second game of the series Tuesday. ... The A's claimed RHP Nick Tepesch off waivers from the Los Angeles Dodgers earlier in the day and designated LHP Eric Surkamp for assignment to create room on the 40-man roster. Tepesch was assigned to Triple-A Nashville.