"The fire is back in the belly [to box] and obviously I want to have a pretty good year this year in regards to rugby," Williams said.
"I know this will prepare me not just physically but mentally as well for a massive season."
Williams, who has played 23 tests for the All Blacks, including the 2011 World Cup final win over France, switched back to union late last year, appearing in New Zealand's last four internationals of 2014.
He wilted in the final rounds against Botha, but is confident he will have enough in the tank if the bout with Welliver goes the eight round distance.
"There's been a lot of work on defence, just getting a good seven, eight week preparation under my belt, with a lot of sparring and training," Williams said.
"That's definitely put me in good stead, fitness-wise. I've got no concerns about being able to go into the late rounds."
He said a fight with Gallen (2-0) was realistic if both win tonight.
Gallen fights mixed martial arts exponent Randall Rayment (1-0).
He won two brief and wildly entertaining fights in which he threw caution to the wind, after getting tagged early in both bouts.
"It was sort of like the red mist fogged over and he went for it," Gallen's trainer Graham Shaw said.
"I've got to try and cut that back and he's just got to be relaxed and box.
"We want to walk away from this fight happy with ourselves and other people saying 'geez, Paul Gallen looked like a boxer'.
"I'm hoping he's just going to be a little bit smarter."
Shaw said Gallen had sparred with experienced heavyweights Bowie Tupou and Solomon Haumono and was much better prepared than for his last fight, when he spent time in hospital in the week leading up to the bout.
Keeping players on tight leash
Sonny Bill Williams, who steps back into the boxing ring tonight, is apparently allowed to do what he likes under his New Zealand Rugby contract. The NZRU are a liberal lot, who even let captain Richie McCaw fly the skies, while loose forward Liam Messam will also be putting up his dukes up at tonight's
Footy Show
Fight Night. Other professional athletes around the world find their employers less forgiving.
• The big daddy of safety clauses. Footballer Stefan Schwarz revealed he would like to be on the first passenger flight into space when he signed for Sunderland in the late 1990s and one of his advisers had booked a place. The club responded by specifically banning the Swede from space flight in his contract.
• Norwegian Stig Bjornebye, whose father was an Olympic ski jumper, also liked the sport, but EPL club Liverpool didn't. They banned him from going within 200m of a ski slope when he signed for them.
• Wellington Phoenix striker Nathan Burns' (pictured) favourite hobby is water skiing, but he can't indulge. "They'll rip up my contract if I get injured - we're not allowed to skate board, own a motorbike ... that stuff is written into our contracts," the Aussie marksman told the Herald.
• San Francisco Giants general manager Al Rosen included spelunking - otherwise known as cave exploration - in a huge list of banned activities in player contracts after reading about a trio who got lost in a cavern.
• Many people would regard mixed martial arts as prohibitively dangerous, but the UFC contracts see danger everywhere else. Among the things they ban are snowboarding, wakeboarding, bungee jumping and horse riding.
• "Troubled" NFL player Dez Bryant had a clause added to his contract two years ago prohibiting him from attending strip clubs. He is allowed to attend nightclubs approved by the Cowboys however. Lucky guy.
• Sunderland again. Whacky manager Paolo di Canio tried to ban mobile phones, fizzy drinks, mayonnaise, ketchup, American fast food and joking or gossiping with the first team within three hours of kickoff. He didn't last long.
• Werder Bremen banned their players from getting tattoos during the season after someone from a rival team had trouble putting a playing jersey on because his tattoo turned septic.
• Legendary Manchester United manager Alex Ferguson banned new signing Nani from doing backflips for goal celebrations fearing he would suffer a terrible injury.
• Barcelona president Josep Maria Baromeu stated the club's enormous contract with Luis Suarez doesn't prohibit him from ear biting.
-AAP
With additional reporting by New Zealand Herald
