LONDON — The silence at Centre Court made abundantly clear that Andy Murray's time as Wimbledon champion was coming to a close. Out of sorts from the start of his quarterfinal against up-and-coming Grigor Dimitrov on Wednesday, Murray — who in 2013 ended Britain's 77-year wait for one of its own to win the men's title at the All England Club — sailed an awkward backhand slice long to fall behind by a set and a break. The crowd of nearly 15,000, usually so vociferous in support of Murray, sat quietly, perhaps not prepared to believe what was happening.