Brook was coming off the back of a stoppage defeat at the hands of Spence Jr a year ago who ripped his IBF welterweight world title away from him in some style in Brook's back yard of Sheffield.
"The first few spars, I was a bit anxious about it, but after I've been in with big guys and then bouncing off and hitting their hands, I realised I have got no problems".
"I'm here to stay". If you can take the injuries out of the equation, then I think Brook would start as the big favourite, but they are worrying me for Saturday night.
"I love beating the odds and getting in there and performing". I've only gone up 10 pounds from yesterday - I felt strong at the new weight.
"I was in a dark place at Christmas time. The feeling I get from going out there, hearing "All of the Lights" and doing my thing in front of all of the fans, it's fantastic for me, that's my drug and I can't wait to get back in there".
Gennady Golovkin stopped Brook in the fifth round when the Sheffield fighter challenged for his world middleweight crown in September 2016. The Belarusian is a seasoned competitor at 154lbs, and will feel his experience in the ring with other super welterweights will give him an edge against Brook.
Brook, who made three IBF title defenses before losing to Spence, was immediately into his stride by landing an array of accurate punches in the first round.More news: Zinedine Zidane: 'Real Madrid players have their confidence back'
The 31-year-old, whose eye sockets have both been reinforced by titanium plates after being fractured in recent defeats, stepped up his attack in the second round as he followed up a bruising left uppercut with a straight right that put Rabchenko on the floor.
Those injuries are a major concerns considering Spence Jr targeted Brook's right eye last May, before breaking the left one. Everything was on the line tonight, his career was a line.
But he was never able to get going against Brook, who will now have his sights set on challenging for world honours once more, or arranging a bout against long-term rival Amir Khan.
"Everyone wants to see that fight [with Khan]", Brook said.
Just because we use the same gym, I am not automatically sticking with Amir, but it will be a great fight and one that has to happen.
"We need to sit down, talk about weight and make the fight".
"He's a very good fighter, very clever", said Brook. Brook has passed his test, Khan has to pass his test on April 21.