Lorena Ochoa drives from the fourth tee box at the Canadian Open on Thursday. (Tom Hanson/Canadian Press)

Defending champion Lorena Ochoa drained a birdie putt on 18 to take a one-stroke lead into the clubhouse Thursday in the Canadian Women's Open at the Ottawa Hunt and Golf Club.

The world's No. 1 was a shot up on Annika Sorenstam after putting together a 66 by way of three birdies on the front nine, three on the back and no bogeys.

"I probably hit 18 greens and I don't remember chipping, so that was good too," said Ochoa, who won the event last year at Royal Mayfair in Edmonton. "It was a great day.

"It always feels good when you have no bogeys on your scorecard."

Sorenstam, seeking her fourth win of what she says will be her final season, rode a hot putter to a 5-under 67, spinning six birdies and a bogey for 31 in her opening nine holes and then traded a pair of bogeys and birdies through her second nine.

"I putted really well," said Sorenstam, who won the Canadian Open in 2001. "I got off to a great start and made tons of putts.

"I missed a few opportunities on the [second] nine, but I made a birdie to finish on a high note, so I'm very pleased," she said. "I got off to a great start and made tons of putts."

Sorenstam, 37, plans to leave the tour to marry boyfriend Mike McGee, start a family and attend to her business interests.

Lee predicts course will get tougher

The former No. 1 from Sweden was a stroke up on Hee-Won Han and Meena Lee, both of Korea and both at 68, and veteran Se Ri Pak and Nicole Castrale, who bogeyed nine (her 18th hole of the day), to drop out of a tie for second.

Lee, who had a bogey-free round, sees the course getting tougher from here on.

"I don't think the trend of today — rounds of 5-under or 4-under — are going to continue the rest of the week," said Lee, who won the Canadian Open in 2005. "I like this course, it's a great course, but at the same time it's really difficult.

"I don't think we'll see 11-under or 10-under on the very last day. Maybe 8-under or 7-under will win it."

The top Canadian from the morning group was Sue Kim, 17, of Langley, B.C., with an even-par 72. Lorie Kane finished the afternoon at plus-3.

Michelle Wie, who needs a top-3 finish to earn an LPGA exemption for next year, was in at least a 16-way tie with Kane, nine strokes back of Ochoa.

With files from the Canadian Press