Many believed Brook would struggle or even fail to make the weight after leaping up two divisions to tackle Gennady Golovkin in middleweight last year.
But the Yorkshireman instead looked sensational on the scales outside Sheffield City Hall yesterday as he weighed in at 146.7lbs.
“I’ve done the hard work getting down to welterweight, people didn’t think I’d get there but you’ve seen me here in great shape,” said Brook.
“I went to middleweight and put that extra weight on to fight Golovkin and now I’m fighting the number one contender for my belt back down at welterweight.
“I love a challenge, and here we are again to impress.
“I’m now ready to let some rockets go.”
Brook believes if he beats his mandatory challenger Errol Spence Jr that he will become the best welterweight in the world.
The only other man who could realistically lay claim to that title is Keith Thurman, who beat Dannny Garcia in March to add the WBC title to the WBA title he already held.
Many believe the winner of this fight will go on to meet American Thurman in a winner-takes-all duel.
“Beating Errol Spence Jr puts me at number one spot in the welterweight division,” Brook said.
“Everybody knows then that I’m the man.
“I want to be remembered as the guy to fight everybody, who never ducked anybody and was involved in some great, great fights.
“This fight going to be up there. I think it’s a candidate for fight of the year because we’re both passionate about wanting to win.
“He’s the challenger and I want to keep it in the Steel City so we’re going to get a thrilling fight.”
Brook has compared tonight’s fight to that of Prince Naseem Hamed’s barnstormer against Kevin Kelley at Madison Square Garden back in 1997.
“Our styles are going to gel and there’s going to be drama,” said Brook.
“We’ll both, maybe, hit the canvas. Fireworks from round one. He won’t take a backwards step, and I won’t.
“Spence Jr will give me many problems.
“He’s an ex-Olympian and a very good fighter. Southpaw suits me, the Ingle style. We’ve seen every style.
“I relish it. I love a challenge. I’ve proven these guys wrong before and I’ll do it again. There’s question marks over Spence Jr – he’s never been in (the ring) with a guy like me.”
Spence Jr, who is widely-regarded as the next big thing across the Atlantic, has a flawless record of 21 wins with 18 knockouts.
He said: “I am motivated by everyone who says I am the next great American welterweight.
“I am the top dog in this division.
“He has power, he has size, and I have the same. Of course, there is going to be drama.”
Despite his superb record, Spence Jr will take a huge step up in class against Brook.
The American’s best win to date is arguably that of fellow American Chris Algieri, who has lost to Brook’s fierce rival, Amir Khan.
Asked if he has ever fought opponents of Brook’s level, Spence Jr said: “No. I’ll tell the truth. But I’ll prove myself on Saturday night – I have the skills and the ability. This is a lifelong dream of mine.
“Another man down – all the talking is over with. Saturday night, let’s glove up.”
There’s a host of Yorkshire interest on the card, including Doncaster-born heavyweight Dave Allen’s fight for the vacant commonwealth title against Jamaican Lenroy Thomas.
Dronfield’s Lewis Taylor will look to bounce back from two defeats in his last three outings when he takes on highly-rated light middleweight Jamie Cox.
Barnsley’s Andy Townend goes in seach of the vacant super featherweight commonwealth strap against Jon Kays, and Sheffield’s Kyle Yousaf, Bradford’s Nadeem Siddique and Atif Shafiq from Rotherham are all in action on the packed card.