ATHENS (Reuters) - Greece's leftist Syriza party maintained a slim lead in the latest opinion poll published on Thursday, winning 25.5 percent of votes compared to 25 percent held by its main conservative rivals, the New Democracy party. Syriza's poll lead has shrivelled since its leader, former premier Alexis Tsipras, resigned in August and called for a snap election to win new backing for a bailout programme. (Reporting by Renee Maltezou and Matthias Williams)