A witness in the murder trial of neighborhood watchman George Zimmerman testified on Friday that he saw Trayvon Martin on top of Zimmerman during a struggle that led to the unarmed black teenager's shooting death in a central Florida gated community last year. But Jonathan Good, a former resident at the townhouse complex, told the jury in Seminole County criminal court that he never saw Martin slam Zimmerman's head into the concrete sidewalk, undermining a key element in Zimmerman's defense. "I did not see that," Good told the court under questioning by a state prosecutor about the racially charged case that triggered civil rights protests and debates about the treatment of black Americans in the U.S. justice system. Police did not arrest Zimmerman, who is white and Hispanic, for 44 days. Zimmerman does not deny killing Martin but he says he did so in self-defense after he was attacked and Martin smashed his head repeatedly into the sidewalk. Good was the fourth former neighbor who partially witnessed the death of Martin on February 26, 2012 to testify in the trial. Each has given a slightly different account, but Good is the first to state that Martin was on top during the struggle. Zimmerman, 29, was a neighborhood watch volunteer in the Retreat at Twin Lakes community in Sanford at the time of the killing. He has pleaded not guilty to second-degree murder and could face life imprisonment if convicted. Martin, 17, was a student at a Miami-area high school and a guest of one of the homeowners. He was returning after buying snacks at a convenience store when he was shot in the chest during a confrontation with Zimmerman. Several former Twin Lakes residents have testified for the prosecution that they heard and caught glimpses of the fight between Zimmerman and Martin, and heard cries for help, on a dark and rainy night near a walkway between units in the community of townhomes. Good said he was watching TV with his wife when he heard a noise outside and saw two people wrestling on the ground, with "a lighter-skinned man" on the bottom. He identified the other man, Martin, by his race and clothing. Good initially told police the person on top was pummeling the other in mixed martial arts style, but backed off that account, later saying the person on top was straddling the other man, but his arms might have been holding the other down rather than punching. Asked by state prosecutor Bernie de la Rionda if he saw the "person on top" grabbing the head of the lighter skinned man and slamming it into the concrete, Good replied "No." In opening statements, defense attorney Don West told the court "Trayvon Martin armed himself with the concrete sidewalk and used it to smash George Zimmerman's head ... That is a deadly weapon." Under cross-examination Good was asked why he had clarified his initial description about blows being struck. "That's what it looked like," Good said. But because it was dark outside, Good said "I can't 100 percent confirm that that was happening." Good also said that he was not sure who made the cries for help that several neighbors have described, although he said "the yelling sounded like it was coming from the person on the bottom." Three residents have told the court that they saw someone who appeared to be Zimmerman on top during the incident. Even though several were close enough to hear the struggle, the prosecution has highlighted the fact that none of them heard a crude death threat that Zimmerman says Martin made moments before he shot him. Jonathan Manalo, another former neighbor, also testified on Friday. Manalo said he grabbed a flashlight and went outside after he heard a gunshot. He used his cellphone to take pictures of Martin's body and the bloodied back of Zimmerman's head. Manalo said Zimmerman was breathing hard and staggering when he initially saw him but did not appear to be in shock. "He was coherent," Manalo said. "He was responding to my questions like any other person." Manalo said he asked Zimmerman what happened and the neighborhood watch volunteer responded, "This guy was beating me up ... I was defending myself and I shot him." Prosecutors say Zimmerman profiled Martin, suspecting him of being up to no good, and killed him in an act of vigilante justice. The defense says Zimmerman was doing his job as part of the neighborhood watch and simply trying to investigate something that he perceived as suspicious. The prosecution faces a tall order to win a conviction for second-degree murder, and under Florida law must convince all six jurors that Zimmerman acted with "ill will" or "hatred" and "an indifference to human life." Under Florida's Stand Your Ground law, which was approved in 2005 and has since been copied by about 30 other states, people fearing for their lives can use deadly force without having to retreat from a confrontation, even when it is possible.