DENVER -- Umpires are keeping a close eye on game balls at Coors Field after the San Francisco Giants expressed concern to the commissioner's office that the Colorado Rockies might be using "juiced" balls when they're batting.

Major League Baseball spokesman Pat Courtney confirmed umpires were instructed to monitor the game balls used Saturday night in the Giants-Rockies game.

Courtney said the Giants spoke to MLB about the Rockies possibly using balls that weren't properly stored in the humidor at Coors Field.

The Rockies began storing game balls in a walk-in humidor eight years ago, the brainchild of stadium engineer Tony Cowell, seen here in May 2002. AP Photo/Ed Andrieski

The story was first reported by the San Francisco Chronicle, which said the Giants filed a complaint with the commissioner's office.

"Complaint is too strong of a word," Courtney told The Associated Press. "There was concern raised."

For nine years, baseballs to be used in Rockies home games have been stored in a humidor, which helps keep them from becoming dried out and more hitter-friendly in Denver's thin air.

Before each game, balls are taken out of the humidor and rubbed up by the umpires' attendant. They are then placed in a ball bag and put back into the humidor. After the national anthem, the bag is taken to the Rockies' dugout and those balls are used during the game.

Umpires were instructed Saturday to monitor the pregame process and make sure nothing fishy was going on.

"We got the bag and brought it over here about a half an hour before the game," crew chief John Hirschbeck said after Colorado beat the Giants 10-9 in 10 innings. "Our clubhouse person brought them out. That's what baseball wants us to do over the weekend, so I will.

"There's nothing going on here. This is a bunch of [nonsense], it really is," he added. "They just scored 19 runs. I mean, come on. There's nothing going on here."

Giants spokesman Jim Moorehead said the team mentioned the issue to the commissioner's office before arriving at Coors Field for a key series this weekend. MLB decided to have umpires keep track of the balls going from the humidor into play, and that started Saturday night.

"It's just to keep a constant visual on the process," Courtney said.

San Francisco won the series opener 2-1 Friday night behind ace Tim Lincecum, but the two-time Cy Young Award winner wasn't satisfied with all the balls he was given.

Lincecum had a 3-2 count on Miguel Olivo in the sixth inning when he got a new ball from plate umpire Laz Diaz. Lincecum rubbed up the ball, then threw it back in and asked for a different one.

"I thought in the back of my mind, this ball doesn't feel like it's a buffed-up ball. It doesn't feel like the ball I got a couple of balls ago," Lincecum said Saturday. "In a situation like that you just throw it back and hopefully get a new one, one that feels right."

Lincecum was caught on video saying "juiced ball," and using an expletive.

"When I said it I guess my emotions got the best of me," he explained. "There's speculation and I kind of verbalized it and that's it."

Lincecum had one of his best outings Friday in the series opener between two teams still fighting for the playoffs. He retired his first 15 batters and allowed just one run on two hits over eight innings.

Rockies shortstop Troy Tulowitzki said he joked with Lincecum on Saturday about the controversy.

"I did see him in the weight room," Tulowitzki said. "I said, 'C'mon, you just pitched a two-hitter against us. How juiced can the balls be?' He said some of them feel like they're big, some of them feel like they're small. I don't know, he had some of his best stuff."

Neither manager wanted to get involved.

"I don't know a thing about it. I can't even tell you where the hell the humidor room really is," Rockies skipper Jim Tracy said. "Since I was asked to manage this club nobody's asked me to approve any baseballs, nobody's asked me to rub any of them up, nobody's asked me if these have been in the humidor. It's absolutely none of my business."

Tulowitzki said he's never seen different balls being used when the Rockies are up.

"I don't know what the whole story is, if they think we're changing balls. All I know is, before I go on deck I stand in the same spot every time, right next to the bucket of balls, and it's never being switched," he said.

Giants manager Bruce Bochy said he was concentrating on the pennant race, but having the umpires keep an eye on the balls before they're put in play erases any doubt about foul play.

"We're not thinking about the humidor, we're just trying to win games," he said. "There's a process and you trust the process. That's the way it has to be. If there's any eyebrows raised, it takes that away."