"There's too many people in the dugout," Duncan said. "I'd get too tired."
Winning a few early points in the month-long battle to make the Yankees' roster as a first baseman, Duncan homered on the first pitch he saw Saturday from Phillies right-hander Adam Eaton, slugging a three-run line-drive shot over the left-field wall.
Facing Chad Durbin in the third inning, Duncan followed with a two-run double off the left-field wall, beating the throw into second with an awkward trick slide. Duncan also played a solid first base, scooping a throw out of the dirt and snagging a line drive, among the plays that left his uniform heavily soiled by afternoon's end.
"You don't make too much of it," Duncan said. "In the spring, you're going to have those feelings where you're locked in and it goes away real quick. I really think consistency is a big part. It's one thing you have to work on mentally, trying to repeat good at-bats and good swings."
The 28-year-old Duncan made an immediate impact last year on the big league level, riding an initial flurry of home runs before finishing the year batting .257 with seven homers and 17 RBIs.
Not invited to big league camp last spring, Duncan now finds himself fighting with Jason Giambi, Morgan Ensberg and Wilson Betemit to win manager Joe Girardi's attention.
Duncan said that he doesn't believe any of the contenders will set themselves apart until the third or fourth week of camp. For now, he is just trying to slow the game down and see the ball well.
"I'm trying to get to the point where I can be the player I know I can be," Duncan said. "I feel everything will play itself out from there. There's no reason to force it. I know what I'm capable of if I get to that level."
Girardi said that the right-handed Duncan could help give balance to a heavily left-handed lineup, whether he is playing first base or the outfield. Girardi said that one appealing aspect of Duncan's game is his tireless energy -- Duncan was at Legends Field early on Saturday for extra infield workouts, which might explain why he said his five innings of play felt more like 15.
"The thing about Shelley, he's going to give you everything he's got every moment he's out there," Girardi said. "He adds a boost. He's a shot in the arm. God bless his Mom and Dad, trying to hold him back when he was a little boy."
First A-Bomb of '08: One season after leading the big leagues with 54 home runs, defending American League MVP Alex Rodriguez hit his first home run of the spring on Saturday, a solo shot in the fifth inning that landed among the sunbathers on the left-center-field berm.
The shot, off Clay Condrey, would have been a two-run homer if Derek Jeter was not picked off of second base during the at-bat. Wilson Betemit and Jesus Montero also homered for New York.
Sharing the base: With a cast of characters vying for Girardi's attention at first base, there's sure to be a crunch for at-bats, no matter what the plan is. Perhaps it's to the advantage of the contenders -- Duncan, Giambi, Ensberg and Betemit -- to know that there really is no rotation set in place.
"I don't have anything set," Girardi said. "Everyone's going to play. Obviously as you start to lean toward a couple of guys in the end, they'll probably get the majority of the playing time."
Even with Duncan's five-RBI afternoon, the Yankees are trying to determine if the 37-year-old Giambi can play defense on an everyday basis this season. For the moment, the Yankees will start Giambi at first base every other day and could use him in back-to-back games later in camp.
Girardi said that he has been pleased to see no ill effects from Giambi's torn right plantar fascia, which cost him two months last season. Girardi is no stranger to plantar fascitis, which slowed him near the end of his playing career.
"I've been impressed with his work here," Girardi said. "He went first to home [Friday] and he's in really good shape. He had no problems doing it. Just watching him run, I think he's doing much better."
High praise: Austin Jackson's mission in Spring Training was to open a few eyes. He accomplished that on Saturday, making a diving catch in the eighth inning that impressed a Hall of Famer.
"He moves better than anybody," said Reggie Jackson, no relation to the young outfielder. "He's the best athlete in the organization.
Jackson, 21, is no stranger to the ballparks of the Florida State League, batting .345 with 10 home runs and 34 RBIs in 67 games for Class A Tampa last year. His nifty snag robbed the Phillies' Michael Cervenak to end the eighth inning.
"It feels good hearing that from one of the best players in baseball," Jackson said. "Once you hear something like that, your hard work is starting to pay off."
Left open: Billy Traber and Heath Phillips both pitched well in their bids to become the Yankees' situational left-hander. Traber threw a scoreless fourth inning, retiring lefties Geoff Jenkins and Chris Snelling. Phillips allowed a hit and struck out two in a scoreless sixth.
"It's important, because that's a spot we're looking for someone to grab hold of," Girardi said.
Bombers bits: Ensberg, a converted third baseman, has looked comfortable at first base in Girardi's eyes. "He looks like he's done it before," he said. ... Right-hander Darrell Rasner said he struggled to throw his curveball for strikes. He allowed one run on two hits and walked two in one inning. ... Hall of Famer Mike Schmidt coached first base for Philadelphia.
Up next: Andy Pettitte makes his first exhibition start of 2008, taking on Cole Hamels and the Phillies at Legends Field in Tampa, Fla., on Sunday at 1:15 p.m. ET.
Bryan Hoch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.