The 23-year-old Phatlum, winless in five seasons on the LPGA Tour, had 10 birdies and two bogeys in her afternoon round at rain-softened Columbia Edgewater. The start of play was delayed an hour, and players were allowed to lift, clean and place their golf balls because of the wet conditions.
''Putting was so good,'' said Phatlum, the winner of the unofficial LPGA Brazil Cup in 2012. ''I changed putter for this week and feeling very good. And I made many birdies today, like 10 birdies today. ... I just try my best and I'm feeling very good with the course. I like the course, very nice.''
The 18-year-old Thompson eagled the par-5 seventh hole - her 16th hole of the day - and had five birdies in her bogey-free morning round. She won the 2011 Navistar LPGA Classic for her lone tour title.
''I would say just my iron shots into the green,'' Thompson said about her strongest play Thursday. ''I hit a lot of good ones and I played the par 5s 4 under, so I took advantage of those. Overall, I've just been working on the consistency of my game and working on tightening my shots up.''
Second-ranked Stacy Lewis, returning to play after withdrawing from the Canadian Women's Open last week because of illness, opened with a 67. She won the Women's British Open on Aug. 4 at St. Andrews and had consecutive early season victories in Singapore and Phoenix.
''I felt great,'' said Lewis, who withdrew after the first round in Canada. ''I felt like a different person out there from last week. Just being focused and I knew what my game plan was. I felt great out there.''
Hall of Famers Juli Inkster and Se Ri Pak also were in the large group at 67 that included Michelle Wie, Yani Tseng and Ai Miyazato. The 53-year-old Inkster won the last of her 31 tour titles in 2006.
''Just iron play was good,'' Wie said. ''Just put in a lot of close shots. Didn't really take advantage of par 5s, which I kind of want to tomorrow. It was a good day. It was consistent.''
Natalie Gulbis had a hole-in-one on the second hole and finished with a 68. She used a 6-iron on the 166-yard hole, with the ball bouncing once and falling in. She has seven aces in competition and 14 overall.
Suzann Pettersen, the 2011 winner, also shot 68.
Defending champion Mika Miyazato opened with a 71.
A 72-hole event for the time, the tournament is back at Columbia Edgewater after four years at Pumpkin Ridge. Columbia Edgewater also was the tournament site from 1990-2008.