He trained like an animal.
On top of that, he (or his trainer) figured out that alot of Thai fighters don't have as sharp western boxing skills and took advantage of that. He could kick as hard as the thais and seemed comfortable in the clinch both delivering and taking knees/elbows.
Professional thaiboxers are hard as nails. Notice that he snaps the heads with his kicks and splits foreheads with his elbows, but the Thai guys keep on going
they called him the "Diamond" for a reason...