The defense attorney for Alexander Pring-Wilson, the former Harvard graduate student charged with first-degree murder, attempted to undermine the credibility of a key prosecution witness yesterday, forcing him to revise parts of his testimony from an April pre-trial hearing.

Pring-Wilson faces life in prison for allegedly stabbing Cambridge teen Michael D. Colono during an altercation outside a Western Avenue pizzeria last year.

Colono’s cousin, Samuel E. Rodriguez, took the witness stand yesterday on the emotionally charged second day of Pring-Wilson’s trial.

Rodriguez, an eyewitness to the stabbing and a participant in the fight, testified to seeing Pring-Wilson holding the alleged murder weapon.

Boston attorney E. Peter Parker, who was hired to represent Pring-Wilson after the defendant’s parents reportedly fired his two previous lawyers, attacked Rodriguez’s credibility in a grueling cross-examination yesterday afternoon in Cambridge’s Middlesex Superior Court.

The Pring-Wilson legal team seeks to show that Rodriguez and Colono posed a clear threat to their client, causing him to act in self-defense.

In the April pre-trial hearing, Rodriguez said Pring-Wilson pushed Colono into a “fence.” Yesterday, Rodriguez revised that account, saying that Colono had his back to a “brick wall.”

“A brick wall in your mind is not the same thing as a fence, is it?” Parker asked.

“That is correct. It is not,” Rodriguez said, admitting that his earlier account had been inaccurate.

Rodriguez also said he himself punched Pring-Wilson outside the pizzeria moments before the stabbing. Rodriguez said he hit Pring-Wilson “square on the side of his head” but that the blow had “no effect” on the defendant.

Rodriguez said he later “was able to drop Alex to the ground.” He said Colono—who was wearing construction boots—tried to kick the defendant, but the kick didn’t connect.

Parker, questioning the witness’ account, noted that the muscular Rodriguez, who works as a bouncer, lifted weights regularly until a few months before the fight.

Parker’s cross-examination attempted to portray Rodriguez as combative.

“To you, the fight’s not over until you put a guy on the ground or he’s not getting up,” Parker said to Rodriguez. “Is that correct?”

“Either that or he admits defeat,” Rodriguez said.