I played this (sit and go) tournament and there was this player @xyz. Rating 250 + change. I played this person in round 5. I was stunned to see this guy with a rating that low having 4/4 wins so far.
It was clearly something fishy. I played that person and I lost as well. Now I have lost to many players in many tournaments and not once have I complained of them being cheats. However a person that low rated winning 5/5 is nonsense.
Could be that I am the biggest muppet in the world and a 1 rated player could wipe the floor with me. Okay fine. However to beat the rest of the field as well is not on. This seems to be prima facie cheating involved here.
How do I report the person?
Delete the name from your post. You are not allowed to accuse people in the forum.
Write an email with your suspicions to the staff using the contact email form. https://support.chess.com/customer/portal/emails/new
They will investigate.
Learn more about the anti-cheating efforts by joining the Cheating Forum.

I played this (sit and go) tournament (<1200 3 min blitz) and there was this player [name deleted after @NotMTwain it was unfair to name the person]. Rating 250 + change. I played this person in round 5. I was stunned to see this guy with a rating that low having 4/4 wins so far.
It was clearly something fishy. I played that person and I lost as well. Now I have lost to many players in many tournaments and not once have I complained of them being cheats. However a person that low rated winning 5/5 is nonsense.
Could be that I am the biggest muppet in the world and a 1 rated player could wipe the floor with me. Okay fine. However to beat the rest of the field as well is not on. This seems to be prima facie cheating involved here.
How do I report the person?