During the six-week trial over AT&T’s planned acquisition of Time Warner, Sidley Austin partner Peter Keisler was a constant presence in court, looking on as O’Melveny & Myers’ Daniel Petrocelli mounted a successful defense of the $85 billion deal.

Now, as the AT&T’s defense against a Justice Department antitrust challenge advances into a Washington appeals court, Keisler is set to assume a more formal role.