KANSAS CITY -- Wilson Betemit belted a grand slam, Bruce Chen got his 10th victory and the Royals won a series.

All of those uncommon events occurred on a rainy Wednesday night in the Royals' 6-3 win over the Oakland Athletics at Kauffman Stadium. The Royals won a series for the first time in their past seven attempts, Betemit's slam was the first of his career and Chen cracked double figures in wins for just the second time in 13 years.

The Royals won the final two of three games against the A's, beating 14-game winner Gio Gonzalez on Tuesday night and then 16-game winner Trevor Cahill.

"We did it against two of the best young pitchers in the American League, so that was pretty impressive," Royals manager Ned Yost said. "We beat two good guys."

Betemit's blast vaulted the Royals into a 4-3 lead in the third inning. Coming on a 3-2 pitch from Cahill, it was a soaring drive that arched down the right-field line and into the seats. The Royals had loaded the bases on Mitch Maier's double, Jarrod Dyson's infield single and Billy Butler's two-out walk.

"With a 3-0 lead, you certainly don't want to put the tying run on," Cahill said. "I didn't have any fastball command tonight. The only pitch I could throw for a strike was the curveball."

Cahill got a full count on Betemit, who fouled off three straight pitches before launching a changeup.

"He threw me every pitch, he was around the zone," Betemit said. "He threw me a changeup, middle in, and I swung the bat good. When I saw the ball land in the seats, I said, 'Yes! I've got it.'"

It was the Royals' fourth grand slam this season, the first three whacked by Yuniesky Betancourt. The team hadn't had so many since 2005.

Betemit also had four RBIs in Tuesday night's 11-3 victory for a total of eight in the two victories.

"Baseball is a crazy game, and you never know what's going to happen," he said.

A two-run homer by Butler, a 412-foot drive to dead center field, upped the advantage to 6-3 in the fifth. His 13th homer followed Mike Aviles' single off Cahill's glove.

"Billy hit that ball to center and just crushed it," Yost said. "The air was heavy from the rain and, as hard as he hit it and as big as this ballpark is, I was just hoping it'd go out. It gave us enough cushion to relax a little bit."

The Royals had three home runs on Tuesday night for a total of five in the two victories.

"The home run has been big for us the last couple of nights," Yost said. "It just shows the importance of having some power on your club that you can put some runs on the board in a hurry against an All-Star pitcher."

Chen, stymied in two previous bids for win No. 10, fell behind, 3-0. The A's first run came after Coco Crisp opened the game with a single. He took second on a flyout, then streaked home on Mark Ellis' single. Left fielder Alex Gordon's throw beat Crisp to the plate, but catcher Brayan Pena couldn't hang onto the ball and he was safe.

"The ball was right on the money again," Yost said. "The ball hit right at the lip of the grass and kind of skipped up on Brayan, but it was a heck of a throw."

Then Kurt Suzuki singled home Ellis.

Matt Carson's home run, a solo bomb to left field, came in the second inning on Chen's first pitch to him. But when Chen left after the fifth inning, he had a 6-3 lead. His record reached 10-7; his only other double-digit season was 2005 when he went 13-10 for Baltimore. Thereafter he had arm trouble which led to Tommy John surgery.

Still on the comeback trail after a 1-6 season for the Royals last year, Chen started this season with Triple-A Omaha and then made to Kansas City as a reliever. Ultimately, he took over injured Gil Meche's starting job.

"If you were to tell me that in September I'd have 10 wins, I'd say you were crazy," Chen said. "But a lot of crazy things have happened and I feel really happy about the way I've been pitching."

So does Yost.

"That's a credit to Bruce that he never gave up," Yost said. "I'm proud of Bruce. He deserves it."

Meche, in his new bullpen role, relieved Chen and pitched a scoreless inning with Dyson's help. After catching Chris Carter's line drive, Dyson made a strong throw to first baseman Kila Ka'aihue to double up Rajai Davis.

Blake Wood, Robinson Tejeda and Joakim Soria each added a scoreless inning. For Soria, the save was his 38th of the season and his 31st in a row. He notched his 19th consecutive scoreless inning despite issuing a couple of two-out walks.

"If I hate something, it's the walks," Soria said. "If I walk a guy, I don't like that, but it's part of baseball. I guess sometimes you have to give up something."

But he ended the game by throwing a called third strike past a very unhappy Daric Barton.

Rain delayed the start of the game and held the in-house attendance to a scattering of fans that ignored the steady early-evening showers and then sat out a one-hour wait for the first pitch.