Duff has been a key player for Fulham since joining from Newcastle United in 2009, racking up 94 Premier League appearances.
Martin Jol: "We've still got time.
"He's not the youngest player in the world, so we will do that in the next couple of months.
"Damien is a very important player.
"He's always in the top three - although he's a bit older - with his fitness.
"He's one of the best pros you will ever see.
"He always gives 100%. If Damien is fit we would always like to keep him."