Phil Mickelson charged up the leaderboard at the British Open on Thursday, grabbing the first-day lead and a share of the record for the best score in a major championship.Aided by birdies on 10, 14, 16 and 17 on Royal Troon’s tricky back nine, the 46-year-old American posted a 63, equalling the record low for majors first set by Johnny Miller in the 1973 U.S. Open.Mickelson came within a fraction of an inch of breaking the record outright. His curling 15-foot putt for birdie circled the hole on 18 before settling just outside it.It was still good enough for an eight-under effort and a bogey-free round and put him three ahead of compatriot Patrick Reed and Martin Kaymer of Germany, with a host of players one stroke further adrift on four-under.Reed’s early 66, a round punctuated by an eagle from the fairway on the par-four third hole, set the clubhouse lead for much of the day.A raft of players threatened but fell back throughout the day, with four-under becoming a bar that was hard to clear.Rory McIlroy and Bubba Watson, among others, advanced to four or five-under and then retreated.WATSON’S WOENorthern Irishman McIlroy double-bogeyed the 13th to drop back from four under. Watson fell farther and faster: he was leading the championship on five under until he triple-bogeyed the short eighth, the Postage Stamp.Reed’s eagle showed early on that it was possible to jump in the right direction too.Former champion Louis Oosthuizen emphasised the point when he aced the 14th hole, his tee shots taking two hops and diving out of sight, helping him to finish with a 71.The day began with local favourite Colin Montgomerie hitting the opening drive, his first appearance at the Open in six years and what may well be his last.He carded a double-bogey on the first hole after finding one of Troon’s deep greenside bunkers. But he took one shot back with a birdie on the third and another on the fourth.The Scot narrowly missed a third straight birdie on the fifth, then went ahead and made one on the sixth.On the Postage Stamp, his tee shot landed within six feet of the pin. He made that birdie and added a fifth on the ninth, meaning as he finished his front nine, Montgomerie was tied for the lead on three-under.He fared less well on the back nine but still finished at even-par.