The Scot will enter tomorrow's match in Melbourne as the 7/11 favourite to reach the final, but has never beaten the world number two in a Grand Slam arena.
Castle, a former British number one, believes that the match is a genuine 50/50 contest.
"Fed loves beating Murray," Castle wrote in The Metro. "Their rivalry is captivating, but it rankles Roger there was some sniping from Andy when he was younger.
"Federer feels he did not always receive the necessary respect from the Briton. The bookmakers are quoting Andy as favourite to reach the final, but I view this as 50-50.
"Fed may appear to be a smooth, suave sort but his devastating game is a big threat to Andy's chances of back-to-back slam victories."
Murray holds a 10-9 record over Federer.