What has Murray's tv audience viewing figures been during the US Open?

The whole tournament has been on sky which means less people have access to watch the match, i know during Wimbledon his figures peaked at over 13million against Nadal, obviously the time of the matches in America deter some people from watching as well. I was just wondering if anyone had any idea of how many people in Britain watched his semi-final against Rafael Nadal and how many are expected to tune in tonight to watch the Federer match?