SURPRISE, Ariz. — Oakland Athletics catcher Kurt Suzuki earns his paycheck, and gets his exercise, when Gio Gonzalez is pitching.

Suzuki frequently makes the jog to the mound to offer calming words for Gonzalez when an inning gets rocky.

But when Gonzalez has his stuff working, no one makes Suzuki's job easier.

"His curveball is probably one of the best in the game," Suzuki said. "He's got life on his heater, and now he's including that changeup. It's improving every day.

"It's fun to catch when he's on. Put it that way."

Athletics manager Bob Geren has said Gonzalez possesses the best natural stuff of anyone on his pitching staff. Now the A's are trying to decide if Gonzalez, 24, is ready to translate his potential into regular success as a big league starter.

The left-hander is locked in competition with Trevor Cahill to be the A's fifth starter, a battle that lik ely won't be decided until final cuts are made next weekend.

It became a two-horse race after starting pitcher Vin Mazzaro was optioned to the minors Saturday along with first baseman Chris Carter and outfielder Michael Taylor.

Gonzalez showed flashes of dominance last season but was wildly inconsistent.

He went 6-7 with a 5.75 ERA in 20 games (17 starts). He averaged 9.94 strikeouts per nine innings, a ratio that was the best in franchise history based on a minimum of 90 innings. But he also led the American League with 5.11 walks per nine innings.

"He's had some great games in the big leagues," A's assistant general manager David Forst said, "and he's had some not-so-great games."

"Composure" is the buzz word surrounding Gonzalez, as it has been since he made his major league debut in August 2008. Gonzalez wears his emotions on his sleeve while pitching, which is why Suzuki wears out the sod between home plate and the mound.

"I'm an aggr essive pitcher," Gonzalez said. "I want to get (every) out. I want to show these guys I've got something in the tank. " It's good to have that emotion and feeling, but controlling it is what's going to work for me."

Gonzalez has posted a 3.46 ERA through four spring appearances (three starts).

Geren was pleased that Gonzalez threw his changeup effectively against the Dodgers on Wednesday, using it to climb back into counts after falling behind.

Gonzalez largely has been a two-pitch guy up to this point, but he said he's gaining confidence with his changeup.

"If he can add a changeup, he can maybe go through a lineup without even using his curveball one time," Suzuki said. "He's got such good life on his (fastball), it would allow him to go deeper in games."

Gonzalez has more dominant stuff than Cahill. But Cahill, at age 22, is the more seasoned pitcher after spending all of 2009 in the A's rotation. And he's making his pitch for the No. 5 job w ith a solid spring of his own.

Both have two appearances left. Gonzalez drew Wednesday's start with Cahill relieving him. Geren said he didn't know yet who would start Monday against Seattle.

Of the two, Gonzalez is the only one with big league relief experience. If Gonzalez doesn't earn the starting job, Forst wouldn't say whether the A's would prefer keeping him as a long reliever or have him start in Triple-A.

"He's pitched like he wants to win a job," Forst said. "(But) for us, it's a matter of results and getting consistent quality starts."

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