There are other things going on in that fight. Yoda wasn't trying to Sidious, Sidious was in fact trying to kill Yoda, Also, Yoda was more in tuned with the Force then Sidious was, and at the time of the fight, the force was EXTREMELY unbalanced and going hay-wire (you know, the systematic slaughtering of the jedi across the galaxy messing with it. Also Yoda was not in his prime, that was about 2-3 hundred years ago. The Guy couldn't get around day to day with out a cane, let alone be prepared to face one of the most powerful Sith in the last two thousand years. Finally Yoda was on a time table, he had to beat Sidious quickly, and that changed a lot of his actions. He couldn't really have a fight like the one he had with Douku. Sidious was obviously not the most powerful. Mace Windu owned him in like 3 shots, if Anakin hadn't surprised him he'd probably could take Anakin, and Mace thought himself less then Yoda.