Diaz, coming off a second-place tie Sunday in Sweden in the European tour's Scandinavian TPC, made a long birdie putt on the par-4 first hole -- her 10th hole of the day on the Royal Mayfair course.
'It was just really fun for me to make the 33-footer,' Diaz said. It's been what I've struggled with, so it was nice to see that go in. It puts me in a good frame of mind going into tomorrow.'
Wie, making her first appearance in the event, closed her late afternoon round with a double bogey on the par-4 ninth. She also had a double-bogey 6 on No. 12, a bogey on the par-5 third and a birdie on the par-4 first.
Kelli Kuehne and Kyeong Bae opened with 66s, 2005 winner Meena Lee topped a group at 67, and Paula Creamer and Juli Inkster shot 68s.
Kuehne has missed the cut in eight of her last nine tournaments.
'Have you seen my year?' Kuehne asked. 'Just one day at a time. I've got some things I've got to go work on practice-wise. I'll go out tomorrow, add them up when we're done, and keep going.'
Defending champion Cristie Kerr had a 69.
'I didn't putt as well as I would like to,' said Kerr, the U.S. Open champion. 'I had a lot of chances out there for birdie and I missed a couple of short putts.
'I have a lot to work on, but with all that being said I shot two under and I'm only four back at this point and it's only Thursday, so there's a lot to be positive about at this point.'
Top-ranked Lorena Ochoa opened with a 70 in her first round since winning the Women's British Open at St. Andrews.
'I hit the ball really good, especially with the irons. Not so good with my driver,' Ochoa said. 'It was kind of like an up and down round, but I'm pleased I finished in the red numbers and all that matters is to be in a good position for the rest of the tournament.'
Alena Sharp shot a 70 to top the 15 Canadians in the field.
'I putted really well today on the front,' said Sharp, from Hamilton, Ontario. 'When I wasn't playing so well, I was making great up and downs and great long putts, so I'm very happy with the way I started. It's nerve-racking the first day at the Canadian Open being a Canadian, so I'm really happy.'
Canadian star Lorie Kane matched Wie with a 75.
'I don't think I'm out of it,' Kane said.