* Koepka fires a 65 to move two ahead * Battling Spieth alone in second after a 67 (Adds detail, quotes) May 21 (Reuters) - Brooks Koepka was exasperated with his ball striking but made three birdies on the back nine to charge past hometown favourite Jordan Spieth and into a two-shot lead after the AT&T Byron Nelson third round in Texas on Saturday. Seeking a second PGA Tour win, Koepka posted a five-under-par 65 for a 16-under total at the TPC Four Seasons Resort in Irving, while fellow American Spieth closed with bogeys at 14 and 18 for a 67 to drop back into second place. More...