Darth Sidious manages to hold his own against Master Yoda, and when he has the chance Yoda escapes and realizes he cannot best him. I found it interesting and shocking that a sith, with a limited human life-span, who was trained in secrecy was more powerful than a jedi who has almost a millennia of experience within the Jedi Order. There are three factors, IMHO, that could probably justify this outcome: 1. The Dark Side offers more potent short-term power, Palpatine just recently let his long charade of a kind hearted and meek senator fall, and finally has the chance to tap into his full power. 2. Perhaps Sidious was right, and Yoda was blinded by his arrogance or overconfidence like Mace Windu and the rest of the Jedi. 3. Yoda might have been losing his skill in old age, he did lose to Count Dooku (sort of), although this one seemed less likely after having witnessed his abilities in the Clone Wars mini-series, events that didn't happen too long before.