Tonya Gill and Wendy Doolan were a stroke back, and Dawn Coe-Jones and Kristal Parker-Manzo shot 70s.
Defending champion Annika Sorenstam, coming off a win last week in the LPGA's first major of the year, opened with a 71. Sorenstam went into the final round last year 10 shots off the lead, but stormed back and beat Mi Hyun Kim in a playoff.
Pak, who started on the back nine at El Caballero Country Club, made birdie putts of 9, 6 and 10 feet to go to 3-under through her first five holes, then gave two shots back when she hit her 4-iron into the water alongside the green on No. 17.
She dropped and hit a wedge within 11 feet of the pin on the par-5, 463-yard hole, but then three-putted from there for the double-bogey.
The 24-year-old star, who has won three majors and 10 other titles since coming on the tour in 1998, quickly settled down after the trouble at No. 17, sinking a 12-footer for birdie on the next hole.
She added three more birdies -- and a lone bogey -- on her final nine holes.
'I was sick for a few days and didn't know if I would be able to play in the tournament. I woke up today and felt great, so I was happy to play,' she said. 'I felt great on the range and my mind is really comfortable right now.
'I told my caddie, 'Let's have some fun and do our best today.' '
Pak missed the cut last year in the tournament, which was played at Wilshire Country Club, by shooting a second-round 78 that left her 6-over.
She tied for 18th in her first tournament of 2002, then tied for ninth last week as Sorenstam won the Kraft Nabisco Championship.
Pak said she worked hard over the winter.
'Physically, I knew I had to be stronger, and I am stronger,' said the 5-foot-6 Pak. 'So I had to work out a lot. Mentally, I worked on how to rest and how to enjoy it more out there.
'I worked on putting and chipping, too, and I took two months off in the offseason with my parents and went to Korea.'
Gill, 31, is back trying to make a go of golf after spending most of the past seven years working in sports marketing, mostly for charity golf events, in Nashville, Tenn.
Her first-round 69 at El Caballero was bogey-free, and she made one 6-footer for par, and tapped in for two others. Gill was the early leader in the clubhouse before Pak finished her round.
'I have never led in an LPGA event, and even now I am not in the lead anymore,' Gill said. 'But I think this is only the third round that I have ever played without a bogey.'
Gill, a three-time academic All-American at the University of Kentucky, played in 19 LPGA events in 1995, but made the cut just seven times and earned a total of $13,558. She has played in just four tournaments since then, one each in 1999 and 2001 and two this year, after earning an exemption by tying for 15th in the LPGA's qualifying tournament.
Among those chasing Pak are Lorie Kane and Laura Diaz, in a group with Sorenstam at 71. Karrie Webb was six shots off the lead at 74.
Full-field scores from The Office Depot Hosted by Amy Alcott