Brooke Henderson in contention at congested LPGA Marathon Classic

Canada's Brooke Henderson sits two strokes back of leader Thidapa Suwannapura, who shot a 6-under-par 65 in the first round at the Marathon Classic on Thursday.

Seven players, including defending champion I.K. Kim and former No. 1 Yani Tseng, were a stroke behind at 66. Henderson, and seven others are tied for ninth at 67.

Suwannapura, who has three top-10 finishes in 117 career LPGA Tour events, birdied five of the final six holes, including the par-5 18th at Highland Meadows in Sylvania, Ohio. The Thai player had her last top-10 finish in 2014 at the Kingsmill Invitational. Her best performance this season was a tie for 18th three weeks ago in Arkansas.

"I'm really happy about the way I played today," the 25-year-old said. "I didn't really look at the leaderboard, and did not even expect anything. I have a chance, so let's see how it goes."

Kim, who started her round on the back nine, said conditions were calmer once she made the turn. She birdied five of the final six holes, including the par-4 ninth.

"Not much wind out there. So I took that advantage on the front nine," she said. "I had a good roll, all three birdies in a row. And birdieing the last two par-5's was really fun."