Duran was a very smart fighter throughout his career. At LW he also had great stamina, punching power, and didn't believe he could lose. Whitaker couldn't punch with him and Duran was smarter than Whitaker was when he fought at 135 (and probably throughout Whitaker's career). When Whitaker tried his (illegal) showboating tricks like getting real low or grabbing Duran's legs, Duran would punch through it, he wouldn't stand for that nonsense.
Whitaker was almost dropped by Pendleton, McGirt, and I believe Mayweather did drop him. Duran rough-houses him, beats him up and knocks him out all in 7-9 rounds.