However, Jonathan Good said he did not see anyone's head being slammed into the concrete sidewalk, which Zimmerman has said Martin did to him.
Zimmerman has claimed that he fatally shot 17-year-old Martin last year in self-defense as the Miami-area teen was banging his head into the concrete sidewalk behind the townhomes in a gated community.
But under prosecution questioning, Jonathan Good said he never saw anyone being attacked that way during the fight between Zimmerman and Martin.
"I couldn't see that," Good said moments later while being cross-examined.
Good, the second person to take the witness stand Friday, said he heard a noise behind his townhome in February 2012, and he saw what looked like a tussle when he stepped out onto his patio to see what was happening.
He said he yelled, "What's going on? Stop it."
Good testified he saw a person in black clothing on top of another person with "white or red" clothing. He said he couldn't see faces but it looked like the person on the bottom had lighter skin. Martin was black and was wearing a dark hoodie. Zimmerman identifies as Hispanic and was wearing a red jacket.
"It looked like there were strikes being thrown, punches being thrown," Good said.
Later, under cross-examination, he said that it looked like the person on top was straddling the person on bottom in a mixed-martial arts move known as "ground and pound." When defense attorney Mark O'Mara asked him if the person on top was Martin, Good said, "Correct, that's what it looked like."
Good also said the person on the bottom yelled for help.
During cross-examination, O'Mara got on his knees to recreate the fighting as he asked Good to walk him through it.
Good was in the middle of dialing 911 inside his townhome when he heard a gunshot, he said.
Zimmerman, 29, could get life in prison if convicted of second-degree murder. Zimmerman followed Martin in his truck and called a police dispatch number before he and the teen got into a fight.
Zimmerman has denied the confrontation had anything to do with race, as Martin's family and their supporters have claimed.
Before Good testified, a worker at a video surveillance company that maintains cameras at the townhome community took the witness stand. A prosecutor played two videos from surveillance cameras; one showed what looks like a person walking past a window at the complex's clubhouse, and another showed what looks like someone with a flashlight by the complex's mailboxes.
Greg McKinney said the digital clock on the video is off by 18 minutes, a point O'Mara hammered home by getting McKinney to concede the timing difference was inexact and could be more than 18 minutes.
Jurors already have been shown some of the state's biggest pieces of evidence, including the 911 call featuring cries for help prosecutors believe came from Martin.
On Thursday, a friend of Martin who had been on the phone with him when he was shot testified about what she heard during his confrontation with Zimmerman.
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Copyright 2013 The Associated Press.