Jack Nicklaus, the architect of the difficult 7,023-yard, par-72 layout, struggled to a 74 but actually gained a stroke on the lead. The legendary golfer, who was two shots back after an opening 68, managed to hang around while 12 of the top-15 players from the first round dropped behind him.
'I love tough golf courses. I think it's fun,' said Nicklaus, who is seeking his 11th Senior Tour victory and his first since the 1996 Tradition. 'Probably the only ones having fun are under par.'
Irwin, five shots back at the start of the day, carded a 70 made up of four birdies and two bogeys. His two-day total of 3-under-par 141 is the highest 36-hole score on the Senior Tour since the 1998 U.S. Senior Open.
Ray Floyd had a 2-under 140 through two rounds of the USGA event at Riviera Country Club, but Irwin came back to win with a 1-under 285 total.
'Conditions are showing themselves to be difficult,' Irwin said. 'Players are seeing some teeth in this course. Whether it's the youth of this course or the wind or the heavy air.'
Coyote Creek, which opened in 1999, surprisingly played to a slightly lower stroke average on Saturday (74.197) than in round one (74.376).
Should Irwin survive both the course and the weather to win on Sunday, he would take the lead over Lee Trevino on the All-Time Senior Tour win list with a total of 30 victories on the elder circuit.
Tied with Nicklaus for second at 2-under are Mark Hayes, Allen Doyle, Tom Jenkins and John Bland, each of whom posted 70s, and Sammy Rachels, who had 71.
Hubert Green stands alone in eighth place at minus one.
Jim Colbert, who held the lead for much of the day, plummeted from the leaderboard with two bogeys and a double-bogey over the last three holes Saturday. He is tied at even-par 144 with seven others, including Tom Watson, whose 5-under 67 was the low round of the day.
First-round leader J.C. Snead followed his first-round 66 with an 80 comprised of nine bogeys and one birdie. Although it was his worst round since the 1995 Senior Tour Championship, he is among the players within five strokes of the lead.
Click here for full-field scores from the Siebel Classic in Silicon Valley