GOODYEAR, Ariz. -- They collided at the knees. One player crumpled to the ground. The other walked away as if nothing had happened.

When Grady Sizemore ran into teammate Shin-Soo Choo during a popup drill at the Indians' Player Development Complex earlier this week, poor Choo didn't know what hit him.

"He was probably nervous to have Grady standing over him like Jack Tatum," manager Eric Wedge joked.

Choo had to be helped off to the trainer's room with what turned out to be minor bruising. And Sizemore?

"Grady's fine," Wedge said before rolling his eyes. "Of course."

Yes, of course. Rare is the incident that can throw Sizemore -- the Indians' indestructible, do-everything center fielder and leadoff man -- off his game. And seemingly limitless is Sizemore's potential when he's on the field.

Sizemore is a three-time All-Star, a two-time Gold Glove winner, a Silver Slugger winner and a member of the 30-homer, 30-stolen base club. Yet the Indians feel that he's just scratched the surface of his capabilities.

"Usually, you have an idea of just how good somebody can be," Wedge said, "but I just don't know with Grady. It's a great compliment to him. Anybody who attacks himself and the game the way he does, as mature and smart and tough as he is, there's just so many intangibles with an unlimited amount of ability. That's what makes a great player."

Sizemore, 26, never lets up in the effort to make himself great. Redefining the term "extended Spring Training," he's been making the 45-minute commute from his home in Scottsdale, Ariz., to the Indians' complex since November.

"You're always looking to improve," Sizemore said. "Or at least I am. You're never going to be satisfied. I'm looking to get stronger and get better."

But Sizemore will have to better himself on an unfamiliar stage as this spring evolves. Later this week he'll head to Clearwater, Fla., to train with Team USA in preparation for the World Baseball Classic, which begins on March 5.

Sizemore was somewhat reluctant to accept the invitation to the Classic because of the injury threat and the time he'll miss with the Indians. But he took it as an opportunity to represent both his country and his sport.

"It's tough, because you don't want to put yourself in harm's way," he said. "Obviously, my obligation is to the Cleveland organization to get ready for the season. But you still play the same game. The odds [of getting hurt] are the same. The biggest thing is, you make sure you prepare yourself and your body so that you can compete at a high level at that time."

Sizemore has performed at a high level in each of his first four full Major League seasons, and each one has seen him improve in some area. In 2006, his doubles total jumped from 37 to 53 and his walks total went from 52 to 78. In '07, the walks went up, to 101, and his stolen bases went from 22 to 33. In '08, he established career highs in homers (33), RBIs (90) and steals (38).

Wedge also believes that Sizemore has improved his arm strength in the outfield and his awareness and intelligence on the basepaths.

"He challenges himself," Wedge said. "He tackles a certain part of his game and just does it. There's nothing he can attack that he doesn't attack."

According to the 2009 Baseball Prospectus, Sizemore is only the third player in MLB history to establish single-season totals of at least 30 homers, 30 stolen bases, 50 doubles and 100 walks at some point in his career. The others were Bobby Abreu and Alex Rodriguez.

Sizemore has already notched at least 20 homers and 20 stolen bases in each of his full seasons. Last year he became just the 10th American League player to go 30-30. And beginning this year, people will realistically look for him to become just the fifth player all-time to go 40-40. Only Jose Canseco, Barry Bonds, A-Rod and Alfonso Soriano have done that.

"Anything's possible with Grady," Wedge said.

When the 40-40 talk is presented to Sizemore, well, you can probably guess how he responds.

"When you start thinking about that kind of stuff is when you get in trouble as a player," he said. "I don't stress home runs or those kinds of numbers. I stress wins. If you're trying to help your team win, those kinds of things take care of themselves."

Sizemore knows that he has room to improve. The Indians view batting average as an overrated stat, but it's nonetheless worth noting that Sizemore's average sank to .268 last year, largely because of his .224 mark against left-handed pitching. He appeared to wear down in the second half, and that leads to Wedge's hope to get Sizemore, who played in 382 straight games from August 2006 to April of last season, more days off this season.

But one underrated, incalculable area where Sizemore has improved over the last two years is in clubhouse leadership. Though quiet by nature, he's learned how to speak up and help his teammates. The Indians rave about the job he did helping Choo last year.

"It's something that's come with time and experience," Sizemore said. "It's something that's come easier to me the more I'm around these guys, and it's something [the higher-ups] have stressed to me that they want to see. They didn't force anything on me. They give you the freedom to say what's on your mind if it's going to help the team."

The talk that Sizemore could better assist the team from the middle of the order, rather than the leadoff spot, will continue this season. It won't die until the Indians find a leadoff-capable bat that allows them to move Sizemore down to the No. 3 spot that suits his statistics.

But Sizemore has never been interested in that talk. Nothing -- be it lineup speculation or the World Baseball Classic or a collision with Choo -- is going to throw him off track.

"When I think about the future," he said, "it's tomorrow. It's not years down the road. It's preparing for tomorrow."

And the Indians, who have Sizemore signed through 2012, have a lot of tomorrows to look forward to.