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Well, you might as well say that Yoda is weaker than Palpatine in a duel:D.
But it's interesting to think about what would happen if Yoda went to kill Anakin and Obi-Wan went after Palpatine...
I think it's clear that Palpatine would have defeated Obi-Wan... But the fight between Yoda and Anakin is unclear (although it's obvious that he would lose to the green friend:)).
P.S.: Why are you so sure that Obi-Wan had no chance of winning?
P.P.S.: And at the end of SW III, it was quite sad for the Jedi Masters, who can be said to have been "stabbed in the back." Only Yoda and Obi-Wan managed to survive...
Well, Obi-Wan was just lucky, and Yoda... well, Yoda is very smart :D, this moment also proves his wisdom.