Ryan Burnett has lost to Nonito Donaire in the second defence of his WBA World Super Bantamweight title.
Burnett had fought well for the opening three rounds but was forced to take a knee in the fourth, floored by a back injury that didn't appear to be caused by a punch.
He got back to his feet but gripped his waistline with his right glove and didn't answer the bell for the fifth round - retiring injured and leaving the ring on a stretcher.
There appeared to be problems after the third round, when Burnett could be heard telling coach Adam Booth: "I can't move it. I can't throw a shot there."
The defeat denies Burnett the chance to fight Zolani Tete in the semi-final of the World Boxing Super Series.
He will now be forced to wait until the conclusion of the tournament to attempt to get back to world champion status in the bantamweight class. Burnett will not have another world title fight until late in 2019.
"To go that way wasn't (the way) that I would like to win," said Burnett. "I've got to give it to Burnett. He is an amazing fighter.
"As a warrior and as a fighter, you've got a guy like Ryan. Early on, he just kept coming."
Burnett had begun the fight well, picking his shots and landing several nice jabs before seeming to have Donaire rattled with a vicious one-two in the third round.
However, with 50 seconds of the fourth round remaining, Burnett fell to his knee with the back injury that has put paid to his hopes of hosting Tete in Belfast next year and brought an end to his 19-fight unbeaten run as a professional.
Donaire – who lost to ex-world two weight champion Carl Frampton in Belfast in March – previously held titles at flyweight, super bantamweight and featherweight– and he also an interim champion at super flyweight. He held the WBO version of the world bantamweight back belt in 2011
He hadn't boxed in the 118lbs bantamweight division for the last seven years. He dropped two divisions and eight pounds in weight in order to take on Burnett.