Watch: Callum Smith's horrifying KO of Lenin Castillo on the Joshua vs. Usyk undercard

Callum Smith brutally knocked out Lenin Castillo at the Tottenham Hotspur Stadium on the undercard of Anthony Joshua vs. Oleksandr Usyk.

The 31-year-old caught Castillo with a vicious right hand in the second round, which immediately sent the Dominican to the canvas. The scary sight, however, was Castillo's leg shaking for nearly five seconds while he lay unconscious on the floor.

Watch the knockout below:

Castillo left the arena on a stretcher upon receiving medical attention from the ringside physicians. According to combat sports journalist Andreas Hale, Castillo is now responsive and is on his way to the hospital.

It was Castillo's fourth loss in his professional boxing journey. The 33-year-old was coming off a win over Ronald Gonzalez in July earlier this year. His record currently stands at 21-4-1.

Smith, meanwhile, bounced back strongly from his one-sided outing against Canelo Alvarez. After losing the WBA (Super) and The Ring super-middleweight titles to the Mexican last year, he moved up one division to light heavyweight. He is now 28-1, with his only loss coming against Alvarez.

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Callum Smith wants to become a light heavyweight champion

Anthony Joshua v Oleksandr Usyk Weigh In

Callum Smith is confident he will find a new home at light heavyweight. Ahead of his fight against Lenin Castillo, Smith made it clear that he plans to replicate his super-middleweight success in his new weight class.

"I’ve got the same goals as I did at 168, I managed to do it there, I’m fully confident that I can do exactly the same at this weight. I think a lot of people thought I’d step-up and fight a knockover job, a gimme, but it’s a good fight, it’s a tough test. He’s boxed for a world title and lost on points but he gave a good account of himself. He’s a big 175, looks heavy-handed, so I can’t afford any slip-ups," said Smith at the Joshua vs. Usyk press conference.

Callum Smith won his first major title in 2018. Following his win over countryman George Graves, he captured the WBA (Super) and The Ring super-middleweight titles.

He went on to defend his straps on two occasions before being dethroned by Canelo Alvarez.