Webb, who won the year's first major at the Kraft Nabisco Championship last month, mixed eight birdies and three bogeys in her round -- with all three bogeys coming on her final six holes.
Defending champion Cristie Kerr leads a group of six players knotted in seventh place after an opening-round 69. She was joined at minus-2 by Brandie Burton, Brittany Lang, Patricia Meunier-Lebouc, Ai Miyazato and Jill McGill.
Paula Creamer, currently ranked second in the world, is tied with 10 players for 13th place at 1 under.
World No. 1 Annika Sorenstam, back from a one-week break, bogeyed four holes Thursday on the way to a 2-over 73. She shares 68th place with 27 other players, including current world No. 3 Lorena Ochoa.
Playing in a threesome with Kerr and Sorenstam, Webb completed her first nine holes -- the back nine -- without a bogey. Her first birdie came at the par-4 11th, where she knocked a wedge within 18 feet and rolled in the putt.
At the par-3 13th, Webb made the first of two 40-plus-foot putts for birdie, rolling in a 40-footer to reach minus-2. She chipped in for birdie at the par-4 14th, then made it three in a row when she knocked a wedge within 6 inches to set up birdie at the par-5 15th.
Webb also birdied the par-3 17th on a long putt, this one from 45 feet, to get to 5 under around the turn.
'The conditions of the greens are unbelievable,' Webb declared. 'There's hardly a mark on them. I really like greens that don't have huge tiers and have more subtle undulations, so I really feel like they fit my eye pretty good.'
After sinking a 12-foot birdie putt at the River Course's first hole, Webb was 1 over for the rest of her round. Both of her bogeys came on par fours: at the fourth, she three-putted from 45 feet; and at the eighth, she hit into a hazard off the tee.
Webb was able to make up for those bogeys with birdies at the seventh and ninth holes, closing out her round with an 18-foot putt at the last.
'My last nine holes were a little up-and-sown, but I managed to put up three birdies and three bogeys,' said Webb. 'Five-under was a really good score today.'
Better than the other stars in her threesome, who were a combined even par.
'It's always good when you're playing with two of the best players on tour,' Webb said of sharing her round with Kerr and Sorenstam. 'Obviously their names speak for themselves, and I'm sure Cristie is pretty pleased with her round. I'm sure Annika will want a lower one tomorrow.'
Kang began her round on the first hole with five straight pars before carding the first of three straight birdies when she chipped in at the par-4 sixth. She made the turn at 2 under after stumbling to a bogey at the ninth, then collected two straight birdies at 10 and 11.
Another bogey at the par-4 14th was followed two holes later by her sixth birdie, on a 55-foot putt at the par-4 16th.
'I had a lot of good fortune today,' said Kang, who also chipped in for her birdie at the 10th. 'I thought maybe some of the girls would shoot low. I didn't know it would be me.'
Joining Creamer in 13th place at minus-1 were Virada Nirapathpongporn, Cindy Rarick, Nadina Light, Reilley Rankin, Alena Sharp, Helen Alfredsson, Shi Hyun Ahn, Juli Inkster, Jennifer Rosales and Amy Hung.