Sidious killed his master when he was more powerful and skilled than him as leland chee stated. Also Sidious exploited a weakness he doesn't possess. Sidious doesn't sleep.
That doesn't mean he could have defeated his master in a straight up fight which is what most people consider powerful. When Malak took a cheap shot at Revan when he thought he was more powerful and skilled he even said he regretted not proving it in fair combat because it left questions. Turns out Malak wasn't stronger or more skilled.