South Korea's Ryu and Japan's Saigo share LPGA Chevron lead
Ryu fired a four-under par 68 and Saigo shot 69 to leave each on nine-under 207 after 54 holes at Carlton Woods in the year's first major women's tournament.
"Another good day," Ryu said. "I was just trying for the good spots, for the easy putts, and I wanted to try to make some birdies. That was good for me."
American Lindy Duncan was third on 208 with American Sarah Schmelzel and China's Liu Yan on 209.
After a bogey at the third hole and a birdie at the par-five fourth, Ryu reeled off four birdies to close the front nine, the last a spectacular chip-in from well off the front of the green.
"I just think, wow, it's amazing," said Ryu, who parred her way through the back nine.
Saigo, however, was not to be outdone. She made a birdie at the par-three third, took a bogey at the fifth, answered with birdies at the sixth and par-five eighth and then holed out from the fringe at the par-five 13th to reach nine-under.
"The course condition was pretty hard. It was pretty difficult to make birdies so I kind of struggled," Saigo said. "Two of them were chip-in birdies from outside, so I was pretty lucky."
Both co-leaders are chasing their first major title.
"I think that's a good pressure for me," Ryu said. "I don't have a major trophy in my home, so that's a goal for my life and in golf."
Saigo added, "I just need to focus on my golf game (and) try to control my mental control and hopefully I'll win."
Ryu led by a stroke entering last year's final round but shot 74 and finished fifth.
"Last year the final round for me was not good," Ryu said. "But I learned to be more calm and more comfortable and more slow. This year I wish to be more enjoying the course."
Liu shared the lead at nine-under until a stumble at the par-three 17th, where she found a bunker, then chipped into sloped rough just above the bunker on the way to a double bogey.
Winds tested players in the late afternoon.
"The front nine was so good for me," she said. "The back nine was such a challenge for me because the wind started to be stronger and the pins were so tough."
"Definitely tested my patience out there," Schmelzel said. "I'm sure there will be more of that tomorrow."