Plagueis was not as powerful as Palpatine, which is why once Palpy learned all he could from Plagueis, he killed Plagueis.
Palpy killed Plagueis in his sleep, hence the line in ROTS about how the only person he couldn't keep from dying was himself and his apprentice killed him in his sleep. The reason that Palpy didn't "fight fairly" is because that's not what the sith do. They are only concerned with power, not justice, so any time they fight it would be to destroy. They aren't concerned with making sure its a fair fight, hence why Palpy would kill his master is his sleep, because that's about as unfair as possible. Palpy wouldn't want to test himself, instead he waited until he was strong enough to defeat his master, so he could become the master. Sith win power through deceit and subterfuge, they don't win through the strength of arms.
The idea that the Sith would stand on a field of battle and clash with their enemy was destroyed with the Brotherhood of Darkness. In its place Bane institituted the rule of two. A master and an apprentice. One to embody power, the other to crave it. Basically the master has the power, and he teaches parts of it to his apprentice, but hopefully not enough so his apprentice eclipses him. However when the apprentice does, he kills the master and takes his place. He does this because since now he's stronger, he wants the power for himself. He then takes a new apprentice and starts the process over. And of course the apprentice is necessary because the master doesn't have the ability to live forever, remember its a power only one has attained, and clearly there was so problem with it because Plagueis kept Palpy around even after he had the power to sustain life.
Read Path of Destruction and Rule of Two, they explain all of this a lot better.
