(Adds details, byline) By Tony Jimenez TROON, Scotland, July 11 (Reuters) - World number three Jordan Spieth has pulled out of next month's Olympics because of fears over the Zika virus, officials said at a news conference held at the British Open on Monday. American Spieth's withdrawal means the Olympic golf tournament will be without the world's top four players. Number one Jason Day, second-ranked Dustin Johnson and number four Rory McIlroy had all previously said they would not be competing in the event. More...