 In 1967 at the age of 28, he was asked to sit in for anchor Walter Cronkite to deliver the nightly news.Zenker, working as a Manager of News Programming at CBS at the time, was chosen because a strike by the American Federation of Television and Radio Artists left the network without an immediate substitute.For 13 days, Zenker delivered the "CBS Evening News", telling viewers that he was "sitting in for Walter Cronkite".