On almost any other day, I believe Yoda would have won. But the Force was definately not with the Jedi during ROTS. Their ability to use the Force has dimished since, at least, AOTC. Plus, after Order 66 and the fall of Anakin, I think Yoda knows that he is not destined to defeat Darth Sidious. He knew that, after a thousand years of waiting, the Sith had played them. They couldn't even sense the Sith Lord in front of them, and the "planets were aligned" in favor of the Dark Lords. Psychologically, and even spiritually, Yoda had lost before he even entered Palpatine's chamber. But he had to at least try.