World number one Lydia Ko shot a tournament record 62 on day two to take control of the CME Group Tour Championship at Tiburon Golf Club.
The New Zealander raced to 12 under par in Florida, three shots clear of America's Ryann O'Toole and Korean So Yeon Ryu.
England's Charley Hull, who began the day in second, carded six birdies but paid the price for a double bogey seven on the sixth as she dropped back into a share of sixth.
The 20-year-old, who is chasing her first LPGA title, also had two bogeys in a round of 70 that left her seven under for the week.
Overnight leader Shanshan Feng of China fared even worse with a 73 that saw her tumble 15 places down the leaderboard.
Ko signalled her intent by opening with a hat-trick of birdies and picked up a further gain at the sixth to turn in 32.
Back-to-back birdies at the start of the back nine preceded her only dropped shot of the day on the 12th but Ko responded with five further gains over the last six holes for a stunning inward 30.
Ko would top the Race to CME Globe standings with victory and take a $1,000,000 (Â£809,454) bonus in the process.
"I think the big key mindset for me is that I've been not really thinking about everything that could happen," she told lpga.com.
"For me to win player the year I know I need to win this championship. A lot of things come with that."