Not only was Yoda there to attack Sidious, but Sidious outright warns him that he is about to attack him. To assume Yoda was taken by surprise, is an utterly retarded. We see him put his hand out in an attempt to block it, the novel supports it. There's no reason to argue against it unless you can provide proof, which you obviously can't. It's like me walking up to an enemy and telling them "I'm about to lay you out" and then a 2 seconds later, I punch him in the jaw, and then someone turning around a claiming he was taken by surprise.
I've provided an explanation why Yoda was knocked unconscious the first time, but you're wanting to ignore that explanation in favor of a retarded explanation that contradicts canon.