Williams was identified by USA coach Jurgen Klinsmann while impressing in the Bundesliga.
He told ussoccer.com: "I was surprised. My phone rang and the person said he was Jurgen Klinsmann.
"He is an idol in Germany. He came to the game against Mainz. I thought he was there to see Fabian Johnson. Afterwards, my coach came to me and said, 'The national team coach wants to talk to you.' I was a little bit nervous. He told me I had to get playing time and to find a rhythm, and then I would get invited. I told him I was excited and happy."