Fess up and Open up

Fess up verb - To accept the truth or existence of (something) usually reluctantly.

Open up and fess up are semantically related In some cases you can use "Open up" instead a verb "Fess up".

Open up verb - Cause to open or to become open.

Fess up and open up are semantically related in confess topic. Sometimes you can use "Fess up" instead a verb "Open up", if it concerns topics such as admit truth or reality of something.