A sickening knockout of Sergey Rabchenko inside two rounds delivered such a reminder in emphatic fashion.
While the Sheffield fighter was expected to win his first fight since losing his IBF welterweight crown to Errol Spence Jr at Bramall Lane last year, it was not meant to be so simple.
Rabchenko had only lost twice previously and had been stopped once.
But, on his first outing at super welterweight, Brook demonstrated frightening power to dismiss the man from Belarus with very little fuss.
After a round and a half finding his range, Brook stepped up the pressure.
A sharp uppercut jolted Rabchenko’s head and sent him stumbling back before a big overhand right smashed into the Belarussian’s temple and buckled his legs until he collapsed to the canvas.
Fight over. Brook most certainly back in the groove, and earning the WBC Silver title in the process.
He announced himself onto the super welterweight stage and immediately put the world champions on notice.
Years of boiling himself down to welterweight took their toll –never more so than in his defeat to Spence when he looked ill on stage at the weigh-in.
When he stood on the scales on Friday he looked healthy and in phenomenal shape.
Those extra seven pounds from 147 to 154 seem to have made a tremendous difference and not just aesthetically.
And the likes of WBC king Jermell Charlo and IBF champion Jarrett Hurd, WBA king Erislandy Lara will have certainly sat up and took notice of such a ferocious performance.
As will long-time rival Amir Khan, with the two finally on a collision course after the former Olympian’s decision to sign with the Matchroom Boxing promotion.
Khan – who returns to the ring in April after two years out to face Phil Lo Greco – will certainly not fancy moving up to 154 to face Brook on this evidence, suggesting there is plenty of negotiating to be done on the terms of a future domestic blockbuster.
“I’m going to be a two-weight world champion,” Brook said. “I’m enjoying this new weight and feeling this strength.
“You get so much out of yourself when you’re not completely killing and draining yourself.
“When you are coming in healthy, you’ve got bounds of energy and that’s how you should be.
“I’m the terminator and I’m back. I’m here to stay.”
Doncaster’s Gavin McDonnell delivered a career best performance to comfortably out-point Gamal Yafai in the chief support fight.
McDonnell admitted his career was on the line as he faced highly-rated up and comer Yafai.
Defeat would have proved a major blow to his chances of a return to world title level this year, after he came up valiantly short in his bid for the WBC super bantamweight crown last February.
But the pressure brought out the best in him as he showed tremendous discipline to stick to the game plan and jab his way through the contest.
Adding in hooks to the body and head further undid the game Yafai who was visibly exhausted in the final rounds.
McDonnell took 116-112 (twice) and 117-113 decisions on the scorecards to claim the WBC International title.
Sheffield’s Kid Galahad made an emphatic statement of his own credentials for a world title shot at featherweight by demolishing the awkward Irving Berry in three rounds.
Two hefty left hands toppled Berry, who was given oxygen in the ring afterwards but appeared to be okay. Galahad has his eye on the winner of the IBF title fight between Lee Selby and Leeds’s Josh Warrington in May.
Atif Shafiq won the all-Rotherham battle with Lee Appleyard, whose corner threw in the towel in the fifth round.
Appleyard had been down three times, all to crippling body shots by Shafiq, whose fast hands were the difference maker throughout.
Deontay Wilder successfully defended his WBC heavyweight title with a 10th-round knockout of Cuba’s Luis Ortiz.