ATHENS, Greece — Alexis Tsipras, who won election as Greece’s prime minister in January on an antiausterity platform that he was later forced to abandon, appeared on Sunday to have been returned to power by Greek voters after his main opponent conceded defeat. With a quarter of the vote counted, Tsipras’ left-wing Syriza party led with 35.4 percent to 28.1 percent for New Democracy, the main right-leaning party. The third largest number of votes was for the Golden Dawn, the neo-fascist party, which was polling at 7.3 percent, about 1 percentage point higher than in January’s election.