Basaksehir beat Besiktas 1-0 at the Fatih Terim stadium in the Super Lig on Friday.
Demba Ba scored the only goal of the game with a classy finish on 50 minutes giving Loris Karius no chance after slotting the ball in the bottom right-hand corner.
Ba showed his former side respect by not celebrating the goal.
The victory meant that Basaksehir moved into first place, one point ahead of Sivasspor who dropped into second.
The defeat was Sergen Yalcin’s first as manager and saw the Black Eagles fall seven points behind their Istanbul rivals.
Besiktas could fall further behind in the title race if they do not beat title challengers Trabzonspor next week.
Basaksehir meanwhile, announced themselves as being among the favorites to win the title.
Basaksehir 0-1 Besiktas
Stat: Başakşehir Fatih Terim
Hakemler: Abdulkadir Bitigen, Volkan Ahmet Narinç, Süleyman Özay
Medipol Başakşehir: Mert Günok, Caiçara, Skrtel, Epureanu, Clichy, Mahmut Tekdemir, İrfan Can Kahveci (Dk. 83 Berkay Özcan), Visca (Dk. 90+3 Ponck), Elia (Dk. 72 Azubuike), Demba Ba, Crivelli
Beşitaş: Karius, Lens, Vida, Ruiz, Caner Erkin, Elneny, Diaby (Dk. 69 Ljajic), Hutchinson, Boateng, N’Koudou (Dk. 74 Boyd), Burak Yılmaz (Dk. 81 Umut Nayir)
Gol: Dk. 50 Demba Ba (Medipol Başakşehir)
