Jack Catterall beat Harlem Eubank on a technical decision after an accidental clash of heads saw their welterweight fight at the Manchester Area halted at the start of the seventh round.
Catterall and Eubank came to blows in the sixth round, both sustaining cuts, but Catterall's was visibly worse.
Referee Bob Williams spoke to the doctor early at the start of the seventh round when the fight was stopped, so it went to the scorecards.
The judges had to score seven rounds, even though only six were fully completed, and favoured Catterall 69-66, 69-66 and 69-65 as Eubank lost his unbeaten record in his 22nd fight.
"The fight was going how I expected it to go. I was breaking him down," said Catterall. "It's not the way I wanted to win."
Before the bout, Eubank faced an awkward encounter with bitter family enemy Conor Benn, who said he "didn't want to miss this".
Each team can nominate someone to watch the wrapping of the opponent's hands and Benn went in to represent Catterall.
Benn will rematch against Chris Eubank Jr - Harlem's cousin - in London on September 20 after Eubank controversially won their first fight in April.
The finals of the Riviera Box Cup will be streamed live on Sky Sports on August 17. Stream Sky Sports with NOW - No contract, cancel anytime.
