"I made sure when the deal was done, wherever I went , that something could be put in the contract to allow me to box," said Williams to stuff.co.nz
"For me, I know what gets me going and the proof is in the pudding. In the last couple of years, when I've been playing my best rugby, that's usually down to the fact that I've been doing boxing on the side.
"I've got a massive year coming up. I've got my first Super 15 and I'll be trying to get my foot back in the door with the All Blacks. But for me, this is what gives me the edge."
Williams will miss the Crusaders first pre-season match, against the Queensland Reds but will join the Crusaders immediately after his fight.
Williams easily defeated Ryan 'Hulk' Hogan in his last fight but Scott Lewis will provide a big step up.
While the seven fight boxer has lost four matches, he knocked out former Queensland State of Origin forward Carl Webb.
"People might be saying that I'm taking this as a joke, and you just have to look at my opponent this time around," added Williams.
"He's got the runs on the board. He beat Carl Webb and just speaking to his trainer today, I know he's already started going on hill runs. I've got to be on my game to beat him. I'm someone that doesn't like to be idle. I don't want to sit around doing nothing.
"What better way to get me ready for the Super rugby season than this. It will sharpen me up.
"I wasn't just upset after the fight, I was embarrassed," added Williams.
"I went pretty hard and said I'd never get back in the ring if they were only going to match me with bums. No disrespect to Ryan , he was a good bloke. But I didn't want to fight against blokes of his calibre.
"It didn't only make me look silly but it was also disrespectful to the sport."