Mark Johnson and Mike Reid share second place at 2-under-par 70. Raymond Floyd, the 1992 winner here, stands in a tie for fourth at minus-1. He was joined there by Bob Gilder, Des Smyth and D.A. Weibring.
Aoki got off to a quick start as he rolled in an 18-foot birdie putt at the par-5 first. He faltered to a bogey at the par-4 sixth. The nine-time winner on the Champions Tour got back into red figures with a birdie at the eighth from 10 feet out.
Aoki, a native of Japan, moved to minus-2 when he converted a 20-footer for birdie on the par-4 11th. Aoki, who finished second at this event in 1992, got to 3 under when he birdied the 13th at Valencia Country Club.
However, Aoki stumbled to another bogey on No. 14. He atoned for that error with a birdie from 22 feet out at the 16th before closing with a pair of pars.
'My putting was so-so today,' said Aoki, who leads after the first round for the first time since the 1999 Las Vegas Senior Classic. 'The wind was very difficult this morning and I had trouble controlling the ball. I am happy with my five birdies today and hopefully I can keep it going tomorrow.'
Johnson also opened with a tap-in birdie at the first, but he tripped to a bogey at the second. He fought back with birdies at six and nine to make the turn at minus-2.
On the back side, Johnson stumbled to a bogey at the 10th. The 50-year-old fell back to even-par when he bogeyed the 14th.
Johnson, who is looking for his first Champions Tour title, came back with an eagle from just inside 12 feet at the par-5 15th. After dropping a shot on the 17th, Johnson closed with a birdie at the last to share second place.
'My short game was very good today and I putted really well,' said Johnson. 'The course has changed completely from when I played in the first Pro-Am. It really firmed up.'
Reid began his round like Johnson with a birdie at one and a bogey at two. He bounced right back with consecutive birdies from the third. The 50-year-old dropped another stroke at the fifth.
The two-time winner on the PGA Tour jumped to minus-3 with an eagle at the par-5 ninth. Reid slipped to bogey at the 10th. He rebounded to birdie the 13th to get back to 3 under. Reid bogeyed the par-5 15th to fall into a tie for second.
'One of those days,' Reid quipped. 'I three-putted from a foot-and-a-half on 15. I was proud of my next shot after that. You just have to move on and forget about the last shot.'
Tom Purtzer, the 2003 champion, posted an even-par 72. He was joined in a tie for eighth by Jerry Pate, Don Pooley, Don Reese, Craig Stadler, Mark McNulty and Keith Fergus.
Gil Morgan, the defending champion, opened with a 4-over-par 76. He stands in a tie for 36th.