The 20-year-old English champion and Walker Cup cap, given a sponsor's invitation to the Andalucian Open at Aloha near Marbella, was the only one of the early starters to grab a three on the 561-yard 10th.
Defending champion Lee Westwood, of Worksop, was happy enough to birdie it with an 18-foot putt after finding a greenside bunker in two - while Swede Peter Hanson also made four as he opened his bid for a win that could take him into the world's top 50 and give him a debut in The Masters in two weeks.
Westwood and Hanson, who flew back to Europe after competing in the CA world championship in Miami last week, remained one under after four holes.
Willett shared top spot with 19-year-old compatriot Oliver Fisher, New Zealander Gareth Paddison, Austrian Florian Praegant, Swedes Per-Ulrik Johansson and Peter Gustafsson, Norwegian Jan-Are Larsen and Argentina's Ricardo Gonzalez.
Westwood was playing with Darren Clarke 18 months after their memorable Ryder Cup time together, but Clarke followed three pars with a bogey four at the difficult 13th.
Among the later starters was Jose Maria Olazabal, playing his first event for seven months after problems with rheumatism, and Welshman Bradley Dredge - also needing to win for a Masters place.
What do you think? Post your comments below. Olazabal has made it known he is far from 100% fit and will decide whether to play in next week's Houston Open and then The Masters only after he has seen how his body copes with the next two or four days.
"I'm not concerned about results at the moment," said the 42-year-old, twice a winner at Augusta.
He feared his future might be in a wheelchair during 18 months out in the mid-1990s and he admitted yesterday that much of his latest lay-off has been spent wondering if he would play on Tour again.
"When you have pain and a total lack of mobility you hit bottom, for sure," said Olazabal.
"But I miss competition; I miss putting myself to the test and I still want to play. It's been an important part of my life."
That includes the Ryder Cup, of course - but having not hit a shot in the qualifying race until today, he accepts his role at Valhalla in September is almost certainly going to be as Nick Faldo's assistant and not, as he originally hoped, as a member of the team again.
Willett added a birdie on the downhill 412-yard 12th to move to three under - where he was joined by Gonzalez, Praegant and Australian Matthew Millar.
Westwood was still only one behind following his second birdie of the day on the 399-yard 15th, while Hanson remained one under and Clarke one over.
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