Austin Ernst set the early pace in the CME Group Tour Championship event in Florida.
The 23-year-old from South Carolina fired an opening round 66 to lead South Korea's Mi Hyang Lee by a shot in Naples.
A clutch of players were a further shot back after two-under-par opening rounds - Jennifer Song, Gerina Piller, Cristie Kerr, Brittany Lincicome and Jaye Marie Green of the United States and South Korea's Sei Young Kim.
Meanwhile, Lydia Ko shot a 69 to help tighten her grip on the world's top ranking.
South Korean Park In-Bee, needing to finish 12th or better to have a chance of pipping the New Zealand teenager to the year-ending top position, opened with a 71.
Source: PA
