Coming off a playoff victory Monday in the Kingsmill Championship, the 24-year-old South Korean player had a 9-under 135 total at Royal Liverpool.
The nine-time LPGA Tour winner hit all 18 greens in regulation and needed only 28 putts. The 64 is the lowest round in competition at Royal Liverpool, breaking the mark of 65 set by four players in the 2006 British Open.
''A bogey-free day and then bunker-free, too, so that was probably the best round I have ever played in a major,'' Shin said. ''So that is a good thing, today's golf. I missed just one fairway. And on the back nine, it's blowing very strong wind, but I just stay focused on my tempo and my timing with my driver and shots.''
South Korea's Inbee Park was second after a 68. She played alongside Shin.
''I had a lot of easy birdies on the back nine, and the front nine was really tough because the wind picked up,'' Park said. ''I've been playing the front nine really hard because my ball is a little bit right to left shape and the wind is coming right to left, so I just have to watch the front nine a little bit more the next two days. The back nine, I've been playing great and I've been having a lot of birdies on the back nine, so feeling really confident with the back nine.''
''I'm surprised to be six shots behind at 3 under,'' Webb said. ''But I'm happy with the way I played and handled the golf course. I'll just try to narrow the gap and see how it goes.''
Play was called off Friday because of high wind and the round was restarted Saturday. The final two rounds are set for Sunday.
Shin opened on the par-5 10th with a 30-yard chip for eagle and birdied the next three holes. She also birdied the 16th for a 6-under 31 on the back nine. She added two more birdies on the front nine.
''I chipped in from 30 yards for the eagle,'' said Shin, sidelined for two months this year after having surgery on her left wrist. ''After that I felt really good and hit great shots at the next three holes to make birdies.''
She won the 2008 event at Sunningdale.
''This tournament, very special for me, because I won in 2008 and it changed, huge, my life,'' Shin said. ''When I won in 2008, I was not a member of the LPGA, so after that, I got a tour card on the LPGA, that was the biggest thing. And also, normally before in 2008, my main goal was KLPGA, and normally I'm watching the LPGA on the TV. So I'm all the time dreaming about this tour.''
American Katie Futcher was 2 under after a 71.
Lydia Ko, the 15-year-old amateur coming off a victory three weeks ago in the Canadian Women's Open, was 1 under along with American Vicky Hurst, Sweden's Carin Koch, Japan's Ai Miyazato and South Korea's Jenny Shin.
Ko, the youngest winner in LPGA Tour history, had a 71. The South Korean-born New Zealander won the New South Wales Open in Australia in January and took the U.S. Women's Amateur last month.
''It would be nice to finish leading amateur,'' Ko said. ''All in all, I just have to try my best. It's the world's best out here, so I can't expect something amazing. You know, I've done well, so all I've got to do is just stay confident.
Jenny Shin shot a 68, Koch had a 71, and Hurst and Ai Miyazato shot 72.
Michelle Wie was 1 over, following her opening 75 with a 70.
''It's amazing how different it was today,'' Wie said. ''We were talking out there about how many stroke difference it would have been if we played yesterday. But it was perfect for me today. It was nice.
Two-time defending champion Yani Tseng was nine strokes back after a 72.
The cut was reduced from 65 and ties to top 50 and ties.