Yoo and Ryu opened with 6-under-par 66s Thursday at the CME Group Titleholders to move into a share of first place with Suzann Pettersen.
Yoo won her first major, the Kraft Nabisco Championship, in the spring. Ryu won the U.S. Women's Open as a non-member of the LPGA last year. She joined the tour this year and won the Jamie Farr Classic in August. She has clinched LPGA Rookie of the Year honors.
Yoo’s round could have been even better. She was running away at 9 under before making double bogey and bogey to close out her round.
“I’m a little disappointed,” Yoo said.
While Ryu wasn’t playing the LPGA last year, she was paying attention when fellow countrywoman Hee Young Park won the CME Group Titleholders.
“I want another South Korean to win,” Ryu said.