The 16-year-old Ko, the winner last year at Vancouver Golf Club at an LPGA Tour-record age of 15 years, 4 months, shot a 5-under 65 on Thursday in perfect conditions at Royal Mayfair for a share of the first-round lead with Angela Stanford and Christel Boeljon.
Ko, the South Korean-born New Zealander, had six birdies and a bogey. She birdied the opening hole, though she figured that might be a bad omen.
"When I start off with a birdie I haven't really played that well, so yeah, I was kind of nervous that I did make a birdie on the first," she said.
She felt some pressure entering the tournament.
"Because you're the defending champion, people are going to expect more," she said. "I called my dad a couple of days ago and he just said, 'Relax. You can't control everything. Just play the game that you want to play.'"
Stanford rebounded after going 0-4 last week in the United States' Solheim Cup loss in Colorado. The Texan birdied five of the first 12 holes in her bogey-free round.
Boeljon had a hole-in-one on the 16th hole. The Dutchwoman also had a bogey-free round, playing Nos. 15-17 in 4 under.
U.S. Solheim Cup teammates Paula Creamer and Cristie Kerr shot 66. Top-ranked Inbee Park, a six-time winner this year, was in a group at 67.
Charley Hull, the 17-year-old English player coming off a strong performance last week in Europe's victory, had a 69 playing alongside Ko.
Second-ranked Stacy Lewis opened with a 74. The Women's British Open winner in her last stroke-play event, Lewis bogeyed five of her first seven holes.
"I played terrible all day," said Lewis, coming off the deflating U.S. loss in the Solheim Cup. "I didn't make any putts. I didn't hit the ball very good and the start just didn't help. I was tired out there. Mentally, I wasn't in it. Just coming off the last week is still tough."
She was asked if she had anything to work on before the second round.
"No, honestly I need to get away from it right now," Lewis said. "Probably the best thing I could do is walk away."