When was the closest Ali came to being knocked out?

Against Earnie Shavers. Ali was ready to go but he smartly resorted to "play-acting", feigning weakness and fatigue, thus confusing Shavers who thought it was one of those Ali tricks so he held back. Remember that at that time, Ali was past his best and was not in shape so just one solid shot from Shavers would've knocked him out. Against Frazier during the first fight, Ali was in tip-top shape and at the top of his game (though not as fast as the pre-exile Ali) and had a lot of pride that you knew he'd get up no matter how many times Frazier knocked him down. Remember that Ali was up in just a couple of seconds so it wasn't close to a KO.