Slaven Bilic’s team saw both their nearest rivals win on Saturday but showed few hints of nerves a day later as they brushed aside Trabzonspor 3-0.
Veli Kavlak gave Besiktas an early lead that was doubled by Demba Ba midway through the first half.
A goal in each half from forward Burak Yilmaz helped Galatasaray to a 2-1 victory against Akhisar Belediyespor, for whom veteran Greece international Theofanis Gekas' late effort was not enough.
Fifth-place Bursaspor were the weekend's big winners as Fernandao bagged a brace in the 5-1 thumping of Kasimpasa.
Ozan Tufan and Volkan Sen got in on the act, meaning Ryan Babel's 86th-minute goal stood as the faintest of consolations.
Onur Ayik scored a 17th-minute opener and Joseph Akpala starred with goals in the closing stages of each half.
Istanbul Basaksehir saw off Eskisehirspor 1-0 and they lie sixth thanks to Semih Senturk's goal.
