The Canadian Brooke Henderson opened a five-stroke lead in the Cambia Portland Classic, shooting a seven-under 65 at Columbia Edgewater in Portland, Ore. Henderson, 17, is trying to join Lydia Ko and Lexi Thompson as the only players to win an L.P.G.A. Tour title before turning 18. Henderson made seven birdies in her bogey-free round and tied the tournament 54-hole record of 19-under 198 set by Yani Tseng in 2013. Morgan Pressel was second after a 65.