Jermall Charlo dropped Williams three times, once in the second and twice in the fifth. Williams, who had not lost a round in 10 consecutive fights, went down for the first time in his career from a strong counter left-hand midway through the second round.
Williams, who established his counter right early, performed well for the next two rounds in the first title fight between undefeated 154-pound champions since Floyd Mayweather dismantled Canelo Alvarez in 2013.