LONDON — For 34 matches over 4½ months, on hard, clay or grass courts, Serena Williams was unbeaten — and, in the minds of many, unbeatable. So it was apt, somehow, that the longest winning streak in women's tennis since 2000 would end at this memorably unpredictable edition of Wimbledon, where up is down, where seedings and pedigree mean nothing whatsoever, where even five-time champion Williams looked lost at the start and, most surprisingly of all, the finish of her fourth-round match. Stumbling on the Centre Court grass a couple of times while her game slumped in crunch time, the No.