(Reuters) - Rory McIlroy believes becoming golf's world number one has lifted his game to new heights and that he will be hard to beat at next month's Singapore Open. The 23-year-old Northern Irishman heads a quality field at the $6 million tournament from November 8-11 that also includes American Phil Mickelson and three-times winner Adam Scott of Australia. World number 12 Louis Oosthuizen ...