SEVILLE, Spain, May 3, 2012 (AFP) - The 43-year-old American, who triumphed at Oak Hill in 2003 to become a Major champion, took advantage of improved afternoon weather conditions to hit eight birdies and three bogeys.
It left him one shot ahead of home favourite Jorge Campillo and the English pair of Robert Rock and Danny Willett, with their compatriot Matthew Baldwin alone in fifth on three under.
Italian Ryder Cup star Francesco Molinari was the best of the morning starters and had to contend with strong winds and heavy rain, as he signed for a two-under-par 70, which was matched in the afternoon by his compatriot Matteo Manassero.
Micheel turned in 33, struck his approach at the 13th to three feet, holed a five-footer at the 15th and two-putted the long 16th from 25 feet to reach six under par.
But he came unstuck at the last, finding sand from the tee, hitting his approach into another bunker and two-putting for bogey.
"I didn't expect this," said Micheel. "Yesterday in the the pro-am I didn't make any birdies, and it wasn't for lack of effort, I can assure you.
"I had a terrific start with four birdies on my first six holes. I think the key was I kept the ball in play, made a couple of nice putts, and I made all my short putts.
"My iron play was good and I just managed myself well and gave myself a lot of opportunities.
"Outside my bogey in the last, I'm extremely pleased. I made eight birdies. It felt great, it really did. I'm ready to play tomorrow in the same conditions. I do enjoy playing in difficult conditions."
Englishman Rock, conqueror of Tiger Woods and world top three Luke Donald, Rory McIlroy and Lee Westwood in Abu Dhabi in January, eagled the 13th and 16th on his return to action after more than a month off.
That break came after his failed attempt to climb into the world ranking's top 50 in time for the Masters, but it is the leading 60 who qualify for next month's US Open and he is currently 60th.
Collated first-round scores from the Spanish Open on Thursday (GBR and IRL unless stated, par 72):
67 Shaun Micheel (USA)
68 Robert Rock, Jorge Campillo (ESP), Danny Willett
69 Matthew Baldwin
70 Sam Hutsby, Francesco Molinari (ITA), Gary Orr, Matteo Manassero (ITA), Rafael Cabrera Bello (ESP), Graeme Storm
71 Craig Lee, Soren Kjeldsen (DEN), Christian Nilsson (SWE), Thomas Aiken (RSA), Marcel Siem (GER), Simon Dyson, Peter Lawrie, James Morrison, Pablo Larrazabal (ESP), Ricardo Gonzalez (ARG)
72 Robert Jan Derksen (NED), Alex Haindl (RSA), Jack Hiluta, Chris Wood, Anders Hansen (DEN), Augustin Domingo (ESP), Nicolas Colsaerts (BEL), Warren Abery (RSA), Kenneth Ferrie, Thomas Norret (DEN), Marc Warren, Fabrizio Zanotti (PAR), Benjamin Hebert (FRA), Bradley Dredge, Gregory Havret (FRA)
73 Soren Hansen (DEN), Joakim Lagergren (SWE), Thongchai Jaidee (THA), Emiliano Grillo (ARG), Gary Boyd, Raul Quiros (ESP), Michael Jonzon (SWE), Adrian Otaegui (ESP), Sam Little, Simon Thornton, Richard Bland, Jeev Milkha Singh (IND), Jamie Moul, Richie Ramsay, Gareth Maybin, Phillip Price, David Lynn, Alvaro Quiros (ESP), Andy Sullivan, Romain Wattel (FRA), Thorbjorn Olesen (DEN), Gregory Bourdy (FRA), Borja Etchart (ESP), Lorenzo Gagli (ITA), Bernd Ritthammer (GER)
74 Carlos Del Moral (ESP), Mikael Lundberg (SWE), James Kingston (RSA), Ricardo Santos (POR), Alfredo Garcia-Heredia (ESP), Lloyd Kennedy, Johan Edfors (SWE), Jose Manuel Lara (ESP), Daniel Denison, Alejandro Canizares (ESP), Markus Brier (AUT), Edoardo Molinari (ITA), George Murray, Peter Whiteford, Matthew Zions (AUS), Rhys Davies, Simon Khan, Lee Slattery, Fredrik Andersson Hed (SWE)
Selected
75 Jose-Maria Olazabal (ESP), Michael Campbell (NZL)
76 Colin Montgomerie, Gonzalo Fdez-Castano (ESP)