By Sudipto Ganguly LONDON (Reuters) -Rafa Nadal’s hopes of completing a rare calendar year Grand Slam ended on Thursday when the Spaniard pulled out of his highly-anticipated Wimbledon semi-final showdown against Australian Nick Kyrgios with an abdominal strain. Nadal won the Australian and French Opens back-to-back this year for the first time in his career and was bidding to become the first man since Rod Laver in 1969 to complete the calendar-year slam. “Unfortunately, I have to pull out from the tournament,” a dejected Nadal told reporters at a hastily arranged news conference, less than 2…