Paula Creamer posted a 4-under-par 68 and stands in a tie for fourth with Becky Brewerton and Christina Kim.
Annika Sorenstam, the top player in the women's game, returned to action for the first time in three weeks. She managed an even-par 72 at Evian Masters Golf Club and stands in a tie for 23rd.
The Swede opened with a birdie on the first, but faltered to a bogey on the fourth.
At the 10th, Sorenstam converted a birdie putt. She parred her next four holes before stumbling to a double bogey on 15 after hitting her tee shot out of bounds. The 34-year-old birdied her next two holes to get to minus-1, but bogeyed the last.
'It is not a good score,' Sorenstam said bluntly. 'I am very, very disappointed.'
Fifteen-year-old Michelle Wie, coming off a quarterfinal appearance at the U.S. Men's Amateur Public Links championship last week, posted a 3-over- par 75 and is tied for 45th. She carded one birdie, two bogeys and a double bogey over the front nine. Around the turn, the young Hawaiian managed a birdie and a bogey.
Koch, who finished second here in 1996, birdied the second from 15 feet out to open her round. The Swede drained back-to-back birdies from the seventh to move to minus-3.
She parred four straight around the turn, then birdied the 13th. Koch closed her round with consecutive birdies from the 17th.
'I had a good day and hit the ball very well,' Koch said. 'I think I only missed two greens and I don't think I missed a fairway. I hit it pretty consistent. I had a lot of birdie chances and managed to make a few of them.'
Brooky quickly moved to minus-2 with a chip-in birdie on the third and a 3-foot birdie putt on five. She tripped to a bogey on No. 8, but came right back with a birdie on nine from 6 feet out.
Brooky made a 9-foot birdie putt on the 11th, then moved to minus-5 with back-to-back birdies from the 13th.
She bogeyed the par-5 15th. Brooky got that stroke back with a birdie at the next. She closed with a 6-foot birdie putt at the last to share the lead.
'When you tee off at the Evian Masters, you know you are going to have smooth greens because the greens here are very nice to putt on,' said Brooky. 'The footprints are not there.'
Baena got off to a fast start with an 8-foot birdie putt on the first. After a pair of pars, she sank a birdie putt on the fourth. The Columbian closed out her first nine with a birdie from 6 feet out on the ninth.
The 28-year-old climbed to 4 under with a kick-in birdie on the 12th. Baena birdied the par-5 15th after pitching her third to 2 feet. She made it two straight as she also birdied the 16th. She closed with a pair of pars to close out a bogey-free round.
'It was very, very solid round,' Baena said. 'I didn't make any long putts. I only missed one green, which I got up and down. I had a lot of opportunities for birdie that I didn't make. I had another four birdie opportunities that were inside of eight feet, but I just didn't make those putts. So it's just exciting, I'm playing really well right now.'
Beth Daniel, Kirsty Taylor, three-time runner-up here Maria Hjorth and two-time champion Laura Davies share seventh place at 3-under-par 69.
Wendy Doolan, the defending champion, carded just one birdie en route to an 8-over 80. She stands in a tie for 73rd.