I assumed Madara won the first battle with Muu because we didn't see him, only Oonoki. Rereading it it seems more probable Muu and Madara only had a short clash... I can imagine Oonoki demanding to protect the honor of stones or whatever, so Muu leaves him.
Don't mistake what I said... Muu would wipe the floor with EMS Madara imo in a full-on, high stakes battle. He probably recognized Madara's abilities and didn't want to get involved in that kind of intense battle (especially when he had rivals like the Second Mizukage to deal with), or Oonoki wanted to fight Madara so the cold-hearted bastard Muu let him. Love that guy.