Very clear in her rearview mirror is world No.1 Nelly Korda, who is also vying for the $4 million top prize.
Korda, coming off her seventh win of the season last week, opened with a 72 and was eight shots behind. She found a simple swing thought after the round and was back to her old self with a 66 that cut the deficit in half with 36 holes left to play.
"Golf is just crazy," Korda said. "You go from playing so well last week to not being able to find the centre of the clubface yesterday. Always humbles you, but that's what you love so much about it.
"Went to the range after my round yesterday. Tried to find a different feel. Felt a little better out there today, and hopefully I can keep progressing."
Yin followed that 30-foot par putt with a 40-foot birdie putt on the next hole. She also holed a birdie putt from about 35 feet on the 11th hole that put her in the lead for good.
She was at 10-under 134. Hye-Jin Choi (68) and Narin An (72) are eight under.
Hannah Green is six off the pace and tied 14th at the season-ending CME Championship. (Dan Himbrechts/AAP PHOTOS)
Recently crowned Greg Norman Medal winner Hannah Green produced a 71 after her opening 69 to be six off the pace at four under.
The Perth native, who is tied 14th, is Australia's only realistic hope with Grace Kim (74), Gabriela Ruffels (74) and Minjee Lee (76) all going backwards.Â
Korda, who already has captured her first award as player of the year, was tied for fourth at 138 with four other players who are either major champions or have been No.1 in the women's world ranking - Jeeno Thitikul, Ayaka Furue, Ruoning Yin and Amy Yang, the defending champion at Tiburon Golf Club.
Furue also is in a tight battle for the Vare Trophy for the lowest scoring average, and she pulled within a fraction of a point of Haeran Ryu.
Yin's round had enough bogeys to slow her momentum, along with a discussion with a rules official over where she took her drop after going in the water on the par-3 fourth hole.
"It looked like I took an improper drop where I went up closer than I should have, where I should have dropped further back," Yin said, who was asked to review footage. "They thought I didn't take the drop properly. I explained to them that I believe I did take the drop properly."
She said it was discussed with everyone in her group. She said a marshal never volunteered any information. The drop stood, she took bogey and moved on.
"I believe my drop was right," Yin said.
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