KANSAS CITY, Mo. -- The Oakland Athletics looked like a team battling for a playoff spot Monday night, and the Kansas City Royals looked like a team checked out and ready for the offseason.

It was supposed to be the other way around.

Khris Davis and Marcus Semien hit three-run homers, the lowly Athletics matched a season high with 17 hits while drawing 10 walks, and they trounced the Royals 16-3 in their series opener.

"It's great to have a game like that," A's manager Bob Melvin said. "For a team that struggles to score runs, it's nice to have a positive mood in the dugout."

In the opposite dugout, the reigning World Series champions could only rue another game slipping away. The Royals (74-69) began the day four back of the final wild-card spot with 20 games to go, and hoped an eight-game stand against Oakland and the White Sox would allow them to gain ground.

But with five teams standing in their way, time is running out.

"These games are easy to turn the page, a lot tougher is losing a one-run game," Royals manager Ned Yost said. "We just got beat. We didn't play good, we didn't pitch good and we got beat."

Daniel Coulombe (3-1) earned the win in relief of Ross Detwiler, though it was the Oakland offense that led the way. The A's scored their most runs this season for their largest margin of victory.

Royals starter Dillon Gee (6-8) was battered for five runs on five hits and four walks in 3 1/3 innings, though he wasn't the only pitcher to melt down. He was followed by three relievers in a span of four batters in the fourth inning, when the A's marched nine to the plate and scored three times.

"When you're out there trying to find something in a big league game against big league hitters, it makes it real tough," Gee said. "They scored a lot of runs, but it's just one game."

By the end, everybody in the A's starting lineup had a hit. Stephen Vogt was 3 for 3 with two walks and two RBI, while Joey Wendle, Arismendy Alcantara and Ryon Healy drove also drove in two apiece.

"We were swinging at a lot of good pitches," said the A's Brett Eibner, who began the season with Kansas City. "We swung at pitches we want to hit and when you do that, the runs seem to pile up."

Davis delivered the first big blow in the third inning when he splashed his 36th homer into the fountains in left field, a shot estimated at 447 feet.

When the Royals matched him with three runs in the bottom half, the A's bounced right back to score three more: Bruce Maxwell and Eibner opened the fourth with back-to-back singles, Wendle hit a sacrifice fly, and the A's churned out four hits and two walks in taking a 6-3 lead.

It was 8-3 when Semien went deep in the sixth, and Oakland merely piled on from there.

"There's not much to say," Yost said. "Just one of those nights."

BIG LEAGUE DEBUTS

Royals OF Hunter Dozier and A's INF Renato Nunez and OF/INF Matt Olson made their major league debuts. Nunez and Olson were brought up from Triple-A Nashville prior to the game, with Olson taking the spot of departed DH Billy Butler on the 40-man roster.

"In Olson's case his parents were here," Melvin said, "so it was nice to get him in the game."

SALVY'S BABY BOY

Royals C Salvador Perez was scratched from the starting lineup after his fiance gave birth to a boy on Monday. His name is Johan Salvador and the Royals said mother and baby are doing well.

TRAINER'S ROOM

Royals RHP Kris Medlen (right rotator cuff inflammation) will not return this season, manager Ned Yost said. LHP Jason Vargas (Tommy John surgery) is also unlikely to pitch for Kansas City this season after making three rehab outings for Triple-A Omaha.

UP NEXT

RHP Jharel Cotton makes his second career start for the A's after beating the Angels in his debut last week. LHP Danny Duffy goes for his eighth win since the All-Star break for Kansas City.