The Filipino GM, who now plays for the United States and ranked eighth overall in the world, suffered the defeat after winning against Samuel Shankland and Daniel Naroditsky in the first two rounds.
Sevian, meanwhile, bagged his first victory of the tournament after losing to Varuzhan Akobian and drawing against Gata Kamsky.
Sevian, the youngest GM in US history, played with the black pieces, a huge disadvantage according to a Business Insider report. He came through, however, as So made a blunder with his clock winding down.
So previously played for the Philippines in various international competitions before moving to the US and switching federations last year