Tszyu's manager Glenn Jennings on Monday revealed Wednesday night's high-stakes showdown with Inoue in Sydney will almost certainly be the Australian's last before finally facing off with Castano early next year.

But all bets will be off if the Japanese ironman ends Tszyu's 19-0 start to his professional career at Qudos Bank Arena.

Tszyu is the mandated No.1-ranked challenger to Castano's WBO light-middleweight belt, but a loss to Inoue would send the 27-year-old back to the drawing board.

"You can call it a gamble. But when you're talking about Tim Tszyu, I don't think anything's a gamble. I'm always confident he's going to get a win," said No Limit Boxing promoter George Rose.

Inoue (17-1-1) is in peak physical condition and looms as Tszyu's fiercest test yet.

In his only defeat, Inoue went the distance with undefeated WBO junior middleweight champion Jaime Munguia before losing his own world-title fight on points to the Mexican in Houston two years ago.

"I've done over 200 rounds of sparring in this camp so I'm feeling super fit and I'm ready to go," Tszyu said on the eve of Tuesday's weigh-in.

"I know it sounds brutal but I enjoy hurting people and I get into the zone and I turn into this different person.

"Two days after that, I'm a different person and I'm nice and relaxed.

"But now I'm in the zone. This is what we do - kill or be killed. It's like back in the gladiator days. This is what fighting and brutality is all about."

Inoue's supremely confident trainer Tatsuya Saita is adamant his charge will pack too many punches for Tszyu.

"Tim Tszyu is an incredible boxer and we respect him very much," Saita said through an interpreter.

"He's also a real gentleman and we appreciate that but I'm sorry to say but having gone through these preparations extensively I do believe that Takeshi is superior to Tim in both power and speed."

But Tszyu is having none of it.

"Speed and power is not the main thing. It's what inside your brain and your IQ and being 10 steps ahead of your opponent," he said.

"You can be the quickest puncher and the hardest puncher. You can do everything perfect but if you aren't able to have the IQ and know what you're doing, it's not going to get you anywhere.

"You never know, this could be my toughest fight. But the preparation I've had and the way I feel in the ring, I feel sorry for Takeshi."

Australian Associated Press