Carl Frampton suffered the first defeat of his professional career as Leo Santa Cruz won on points to regain the WBA featherweight title in Las Vegas.
The 28-year-old Mexican got a deserved narrow points verdict in an all-action contest and afterwards promised to fight Frampton for a third time.
One judge scored it a draw, but the other two favoured Santa Cruz 115-113.
“Leo probably deserved it. It was my fault and I want to apologise to the travelling fans,” said Frampton, 29.
“I hope we can do it again – we need to do it again.”
Thousands of fans had travelled from Northern Ireland to help create a feverish atmosphere at the MGM Grand Garden Arena.
They saw Santa Cruz, a three-division champion, get off to a fine start, using his height and reach advantage better than he had done when suffering his first defeat against Frampton in July.
Frampton, who had won all 23 of his previous pro fights, warmed to his task more after a slow start.
But he found Santa Cruz, relentless with a productive jab, in determined mood to avenge that points defeat in Brooklyn.
Frampton won some rounds back and finished strongly in the 12th, but Santa Cruz held firm to take the belt.
“My feet were a bit slow, I needed to let my hands go a bit more when I got close,” added Belfast man Frampton.
“We had a fantastic training camp, I felt completely prepared. I feel like I’ve let the people down.”
Frampton said he would like Santa Cruz to come to Belfast for what would be a third meeting between them.
Santa Cruz said: “I worked hard and did what I had to.
“I fought a great fighter and let’s make a third fight. He is a great fighter and a great counter-puncher.
“It was tough to change my style, in my head I wanted to go for it, but I was told to box him and that’s what I did.
“I am a man of my word. We are both great fighters, we deserve it and let’s go again.”
Read more: BBC