Luffy wasn't able to KO Lucci either even though he got a free hit on Lucci by attacking him while he was distracted, used G5 his peak power and used 3 name techniques.
I look at the injuries Lucci sustain from his battles and his condition afterwards. After Lucci battle against Luffy, Lucci was in good condition and only had a small injury on his cheek. After his battle against Zoro, Lucci had three major scars that went horizontal across his whole body, he could barely move and only had the energy to remain standing for short period of time. He didn't even have the power to go get Kaku out of harms way.