Sizemore said he felt good Friday night after going feet first into a sliding bag. He repeated the drill four times.
"I felt fine," Sizemore said, admitting he was a bit apprehensive on his first attempt.
"It was in the back of my mind [to be careful], but after the first one, everything was normal," the 28-year-old outfielder said.
Sizemore had microfracture surgery on his left knee June 4. He got hurt in an exhibition game April 1 and hit only .211 in 33 regular-season games before being placed on the disabled list May 19 after aggravating the injury while sliding in a game in Baltimore.
Sizemore said he is eager to play for the first time in 10 months. He is scheduled to bat twice as the designated hitter on Sunday against the Arizona Diamondbacks.
Manager Manny Acta doubts that Sizemore can get enough at-bats the remainder of spring training to be ready for Opening Day, April 1.
"It's too much to ask," Acta said. "The program we have planned for him just won't allow it. He would have to play every day. We're going to try and get him up to every other day. But's he's ready to DH on Sunday."
Sizemore ran the bases for the first time five days ago and repeated those drills Tuesday before venturing into what probably is the most dangerous roadblock to playing full time: sliding to a stop.
"That's how I hurt it, sliding," said Sizemore, who wore a foam pad over the knee. "I won't have to wear a brace, but we're experimenting with various pads just to see which is most comfortable. We'll test them out."
Sizemore said he has tempered enthusiasm for his progress thus far but will probably be excited if all goes well following his first actual appearance in a game.
"Until then, all this is nice, but doesn't mean as much to me," he said.
Sizemore said that was how he approached rehab last spring, too. In 2009, he played most of the season with an inflamed left elbow that eventually required surgery in September. He was held out of the lineup until midway through 2010 training camp.
"I had to build up strength in my arm, so that helped me understand the process this year," Sizemore said. "I'm probably a little more patient, but at the same time, I'm a little more eager to make up for even more lost time."