Kalu Uche put Kasimpasa in front after 73 minutes before substitute Huseyin Kala sealed victory four minutes later for the host, which ended Fenerbahce's unbeaten start to the season.
Stanislav Sestak's late strike rescued a 1-1 draw for Bursaspor at home to Gaziantepspor.
Ekrem Dag had given the host a surprise 28th-minute lead, one that looked enough for all three points until Sestak levelled five minutes from time.
Finally, Karabukspor eased past Akhisar Belediye 3-1.
Florin Cernat set it on the way six minutes before half-time and further strikes from former Newcastle United striker Lomana LuaLua (69) and Ilhan Parlak (76) wrapped up the win before Emrah Eren scored a late consolation for the host.