Austin Ernst and Lydia Ko impress in first round of CME Group Tour Championship

Austin Ernst fired a six-under 66 to grab a one-shot lead after the opening round of the season-ending CME Group Tour Championship.

While the 23-year-old world no 53 edged into the lead, New Zealand teenager Lydia Ko shot a 69 to help tighten her grip on the world's top ranking ahead of South Korean Park In-Bee, who opened on 71 to share 22nd.

Park must finish 12th or better to have a chance of swiping the year-end top spot from Ko, the defending champion who took the season points bonus and the title last year for a record $1.5m payday.

American Ernst, seeking only her second LPGA title, found form with her putter on Thursday to steer herself to the top of the leaderboard in Naples, Florida.

Ernst said: "There were still two or three putts I feel like I should have made, but to be able to shoot six and say that is a step in the right direction.

"It has been a little streaky. I have been up and down with the putter. I had eight birdies today. When I get around like that with the putter, that's when I have good weeks."

Ernst has four top-10 efforts this season, including a share of second at the Yokohama Tire Classic in August, where she adopted a claw putting style.

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A birdie-birdie finish ensured Ernst leads by a single stroke over South Korean Lee Mi-Hyang with South Korean Kim Sei-Young and Americans Jennifer Song, Cristie Kerr, Jaye Marie Green, Brittany Lincicome and Gerina Piller sharing third on 68.

Ko, the 18-year-old Kiwi star who has five wins this season, carded a bogey at the par-3 fifth but birdied the par-5 sixth and added three more birdies to join a pack on 69 with American Alison Lee and South Koreans IK Kim and Jang Ha Na.