In the time before the Clone Wars kicked off, Master Yoda took a much more active role in lightsaber training and combat within the dojos of the Jedi Temple. He would take fights against any Jedi who was brave enough, and as you can guess, he would win every one.
Most of them were just happy to last longer than 20 seconds against the Grandmaster, and bragged about it to their friends if they could. That was until Master Plo Koon showed up though, and actually managed to defeat Yoda without even using his Electric Judgement ability. Yoda was impressed with Master Plo and gave him great respect from that moment onwards. You can only imagine the reaction of the Padawans watching the fight!