Sebastian Vettel is three points behind leader Lewis Hamilton MERCEDES say they face a difficult task stopping Ferrari’s Sebastian Vettel retaking the championship lead in Singapore on Sunday. Mercedes driver Lewis Hamilton moved ahead of the German for the first time this season with victory in Italy. The Briton leads Vettel by three points as they head to the slow and twisty Marina Bay track this weekend. Team boss Toto Wolff said: “It is a circuit that we have found difficult to master” He added: “We head to Asia with the expectation that we have a big challenge ahead of us.” Mercedes were beaten by Ferrari in Singapore in 2015, when they had their least competitive weekend of…