Uhh I dont think so, there is no way Yoda would have lost to Anakin/Vader. If Obi could beat him, and Obi wasnt strong enough to fight Sideous, then there should be no reason why Yoda could not have beaten Anakin.
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If you read the book, Yoda calls Obi-Wan The Master, I think he was more powerful than Yoda at the end...
Deadly_Nightshade