Fletcher registered for the first time since September to give the Black Cats the lead at Craven Cottage before Sessegnon sealed the points late on with a rasping drive from range.
The Scottish poacher believes that both men can build upon a positive display to move up the table if they continue to work hard in training.
He said: "It was a relief to get a goal and get the three points. It really has been coming.
"Sessegnon has been working very hard and he's been playing brilliantly so it's fantastic."
Victory at Craven Cottage was Sunderland's first away success of the season and only their second win of the campaign to date.