Pat Hurst and Michelle McGann each fired rounds of 3-under-par 69. That duo shares second place with Eva Dahllof, Liz Early, Catrin Nilsmark, Kristal Parker-Manzo, Marisa Baena and Patti Rizzo.
Yeo did not get off to the best of starts during her round at Squaw Creek Country Club. After a pair of pars, she three-putted for bogey at the par-4 third. Yeo got going at the par-5 fifth with a birdie to get back to even- par.
She then notched back-to-back birdies that helped her make the turn at 2- under. Yeo stumbled again to start her back nine. She bogeyed the par-4 10th when she missed the green left and was unable to get up-and-down.
Yeo then caught fire. She atoned for her mistake at the 10th with a birdie at the 11th from nine feet out and made it two in a row with a three-foot birdie putt at No. 12.
After a par at the 13th, Yeo notched back-to-back birdies from the 14th to briefly move to minus-5. However, she bogeyed the par-4 16th before parring the final two holes to finish at minus-4.
'I played really calm and very focused,' said Yeo, who tied her career-best round. 'I think I only missed two greens and I did really good.'
Yeo, whose best career finish was tied for 33rd at the Corning Classic in 2002, was stunned to be on top of the leaderboard with the strong field.
'Well, I'm really surprised and I realized on No. 15 and people were coming around taking pictures so I knew something was going on,' Yeo said. 'As soon as I went to 5-under, I knew today was the day.'
Hurst had an adventurous round. She started on the back nine with a pair of bogeys. On the front side, she birdied the fourth, eagled the par-5 fifth and birdied the par-five eighth to join the group at minus-2.
'It was obviously very sloppy out there,' said Hurst in reference to the course conditions that were sloppy since rain has plagued the area. 'They did an unbelievable job getting the greens in shape so you can putt on them.'
Hurst also began her round on the back side Friday. She parred her first eight holes before bogeying the par-4 18th. Around the turn, Hurst took advantage of the par-5s with birdies at Nos. 5 and 8, both par-5s. She closed her round in style with a 15-foot birdie at the par-3 ninth.
'It seems like they're putting the pins on the back trying to keep the pins on a higher level just because of the wetness from the rain,' said Hurst. 'It's tough to get them close because you're spinning it so much that it's pretty much you seem to have to fly the green to back them up to the hole.'
Rachel Teske, Michele Redman, Dottie Pepper, Joanne Mills, Michelle Ellis, Amy Fruhwirth, Vicki Goetze-Ackerman, Lorie Kane and Kristin Lindstsrom are tied for 10th place at 2-under-par 70.
Annika Sorenstam, the top player in the women's game, is among a large group of players at minus-1.
Defending champion Mi Hyun Kim struggled to a 1-over-par 73. After just one birdie and two bogeys in her round, Kim is tied with a large group of players in 58th place.