(Corrects reference to British Open from this month to last month) By Mark Lamport-Stokes AKRON, Ohio, Aug 1 (Reuters) - Euphoric after winning his fifth major crown at the British Open last month, Phil Mickelson had a tough time mentally re-adapting to the rigours of the PGA Tour at the WGC-Bridgestone Invitational on Thursday. The American world number two, who by his own admission has been playing some of the best golf of his life in recent weeks, carded a two-over-par 72 in the opening round at Firestone Country Club, a stroke worse than the day's average score. ... More...