South Koreas In Gee Chun leads by one shot at the halfway stage of the AIG Womens Open at Muirfield and remains on course for a fourth major title.
Chun, who won the Womens PGA Championship at Congressional in June, hit six birdies to card an eight-under-par 66 in the second round.
South Africas Ashleigh Buhai and Swedens Madelene Sagstrom both trail the leader by one shot.
Sundays winner will claim £903,000 from the record £6m prize fund.
However, 27-year-old Chun said she was also motivated by a personal wager with her caddie.
"Before the start of the tournament my caddie Dean and I talked about the course and had a little bet," she said.
"If I have a bogey-free round hes going to buy me dinner and pay me $100 each day, so before each round its like setting another goal.
"I think that mindset helped a lot on the course; I had two bogeys over the last two days but I want to keep trying for a bogey-free round."
Buhai looked as if she may take the overall lead as she went through the first nine holes in an impressive 30, including an eagle at the fifth, but a bogey on the final hole saw the 33-year-old finish the day in joint second.
She matched the 65 posted by last years joint runner-up Sagstrom, while overnight leader Hinako Shibuno followed her opening 65 with a 73 to fall four shots off the pace.
Scotlands Louise Duncan, who only turned professional two weeks ago, carded a 73 to slip back to two under alongside Irelands Leona Maguire.
English trio Jodi Ewart Shadoff, Charley Hull and Mel Reid are all a stroke further back.