The 25-year-old has had a fine season at Craven Cottage and this week helped Fulham reach the Championship Play-off Final.
However, his future in west London remains uncertain and West Ham have also shown strong interest in the ex-Tottenham youngster.
Fredericks has yet to sign the new deal he was offered by Fulham last year – and will not do so if Slavisa Jokanovic’s men lose to Aston Villa at Wembley because he wants to play in the Premier League next season.
Palace boss Roy Hodgson and sporting director Dougie Freedman watched him in action as Fulham beat Derby 2-0 on Monday to set up the showdown with Steve Bruce’s Villa, who drew 0-0 with Middlesbrough last night to secure a place in the final, 1-0 on aggregate.
Bayer Leverkusen have signed keeper Lukas Hradecky from Eintracht Frankfurt ahead of Bernd Leno's likely exit.