Casey reigns in Spain with sparkling 63
But Sandy Lyle is on the verge of losing the European tour card he has retained for 25 years following a miserable six over 77.
Casey, up to 22nd in the rankings after beginning the year only 104th, is one ahead of Sergio Garcia and two in front of Pádraig Harrington and Robert Karlsson after the first round at Club de Campo.
Lyle, however, is close to last and having run up two double bogeys and three bogeys he said: "That's game, set and match I think I haven't got a cat in hell's chance."
The 45-year-old Scot needs a top nine finish to have any hope of keeping his place among the tour's top 40 career money-winners.
Failure to do so will mean he has to seek special invitations from sponsors in order to play a full schedule next season.
Casey's round did not count as a course record-equalling effort because placing was allowed on the wet fairways. But he was delighted nonetheless.
"I didn't expect much after two weeks off and not hitting it well the last two days," said the 26-year-old.
"I was working on some stuff, but then decided to throw it out of the window and forget about how I was swinging it."
He collected seven birdies and also an eagle at the 518-yard fourth, where he struck a two-iron 240 yards uphill to 20 feet. Garcia had two eagles, first sinking a 103-yard wedge to the fifth and then making an 18-footer on the long seventh.
After failing to qualify for the US Tour's season-ending Tour Championship - only the top 30 money winners play that and he is down in 93rd place Garcia needed to play this week to make sure of appearing in next week's Volvo Masters at Valderrama.
For the first time only the top 60 on the Order of Merit qualify and he is currently 58th.
That looks a real comedown for a player who was fourth in the world entering 2003, but Garcia embarked on swing changes and knew it would be time before he was firing on all cylinders again.
However, such is the nature of golf that when he started hitting the ball better his putting deserted him.
He is 182nd in that category on the US circuit, but had only 28 putts yesterday.
Harrington enjoyed himself again at one of his happiest hunting grounds. His first tour win was the 1996 Spanish Open and three years ago he was successful on the course once more.
It is a different-looking Harrington now, though.
His pre-shot routine includes a succession of waggles that he hopes will help make him feel more comfortable over the ball.
Seve Ballesteros, diagnosed with arthritis in his back last month, looks set to miss yet another halfway cut after a 76. But he is already a winner this week.
Ballesteros collected €1,000 for an 17 under par team score of 54 in the eve-of-tournament pro-am, where his partner included Enrique Ponce, one of Spain's most famous bull-fighters.
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