She led by a shot from teenage Thai star Ariya Jutanugarn, who also got to five under before bogeying her last hole on a still, sunny morning on Melbourne's sandbelt.
Lee has a novel golfing aim -- she wants to be the pick of the Lees.
Which will be no mean feat this week. There are five golfers bearing the Lee moniker at Royal Melbourne -- three Koreans, Min Lee from Taiwan and Aussie teen Minjee Lee.
But they are all chasing Ilhee Lee.
It was a far cry from her first visit to the famed sandbelt layout three years ago, when she missed the cut at the Open.
Scarred by that experience, she intended to skip the 2015 event, only to have a late change of heart.
"Now I know how to play this golf course better than when I last played here," said Lee, who marvelled at how the fairways suddenly looked much wider yesterday.
The Korean had battled a cold early in the week, coming into the tournament with low expectations.
But a great day off the tee and a couple of monster birdie putts at the 13th and 15th holes rocketed her to the top of the leaderboard.
"There are lots of Lees here," said the world No59, whose sole LPGA Tour victory came two years ago in the Bahamas. "I want to be the most famous Lee. That's my goal."
There are 10 Lees in the top 80 on the women's world rankings, which reflects the dominance of players with Korean heritage.
Jutanugarn first leapt to prominence when she earned a spot in the 2007 Honda LPGA Thailand at the age of 11, making her the youngest player ever to qualify for an LPGA tournament.
Canada's Alena Sharp carded a 70 to be tied for third at three-under with Ko and Min Seo Kwak (Korea).
Katherine Kirk and Rebecca Artis were the leading Australians after carding opening rounds of one-under 72.
Karrie Webb began her title defence with a level-par 73.
- AAP