Rees's corner pulled him out with one second remaining in the fifth round of the world title fight, just as the American was starting to punish him with powerful combinations.
The 32-year-old former light-welterweight world champion did admit that Broner is the best fighter he has ever faced.
"He's the best I've been in with. He's world champion for a reason," Rees told Sky Sports. "He's already a superstar.
"I made a lot of mistakes. My skillset's better than that so I'm disappointed in a way.
"He was catching me with some great shots and the gameplan went out of the window. When you're stunned you don't think properly."
It was only the second defeat of Rees's career, while Broner improved his record to 26-0.