Seven-time major champion Inbee Park produced a five-under round on day two of the women's golf competition at Rio 2016 to move a stroke clear at the top of the leaderboard.
South Korea's Park trailed British Open champion Ariya Jutanugarn following the opening day's action, but the Thai faltered on Thursday, staying put on six under to fall four off the lead.
Park registered her only bogey on hole seven and finished with two birdies to ensure she ended the day just clear of American Stacy Lewis.
Lewis stormed into contention by carding an eight-under 63 and sits second, putting a double bogey at 14 behind her to make gains on the last four holes.
"The putter just got hot," Lewis said. "I definitely hit it better, but the putter was on fire. And then I hit some close and made some easy birdies coming in.
"I actually didn't know what I shot at the end of the day. I was just kind of in the zone.
"This course is so tricky off the tees, finding the right lines. Having a competitive round under your belt definitely helps. I just trusted things a little bit more.
"I feel really comfortable on the greens and ultimately that is what it is going to come down to on Saturday [final day]."
Great Britain's Charley Hull and Brooke Henderson of Canada are a shot further back after strong day-two showings, while Marianne Skarpnord (Norway), Nicole Broch Larsen (Denmark) and Candie Kung (Chinese Taipei) are locked in a three-way tie for fifth at seven under.