Fortuna Sittard
After featured for the Jong FC Twente, Thesker was loaned out to Fortuna Sittard for the rest of the season in the January transfer window. He made his Fortuna Sittard debut for the club on 21 January 2011, coming on as a second-half substitute, in a 3–0 loss against Dordrecht. He went on to make 15 appearance for Fortuna Sittard and at the end of the 2010–11 season, he returned to his parent club.