LONDON — World middleweight champion Gennady Golovkin fought off the ambitious challenge to his supremacy from Kell Brook with a fifth-round stoppage on Saturday.

Brook’s corner threw in the towel after the welterweight champion, who stepped up two weight classes to face Golovkin, faded against the formidable Kazakh, who improved his record to 36-0.

Here's Golovkin's @GGGBoxing stopping Kell Brook in rd 5. #GolovkinBrook#Boxingpic.twitter.com/uk1Bz4dm8l

— The Ring Legacy (@RingMagazine_) September 10, 2016

But if the powerful, big-punching Golovkin was expecting Brook to capitulate easily at London’s 02 Arena, it didn’t go to plan. It was a more open, thrilling contest than anticipated, with Brook posing a sporadic threat.

Brook succumbed to the first defeat of his professional career in his 37th fight as Golovkin retained his WBA, WBC and IBF middleweight belts.