By Matt Slater
BBC Sport at Royal Troon
-5 P Casey (Eng), T Levet (Fra)
-4 M Campbell (NZ)
-3 G Evans (Eng), KJ Choi (Kor), C Petterson (Swe), M Goggin (Aus), K Ferrie (Eng), S Wilson (Sco), V Singh (Fij), M Olander (Swe), A Forsyth (Sco)
England's Paul Casey shared the lead with France's Thomas Levet on five under after day one at Royal Troon.
The duo carded rounds of 66 to lead New Zealand's Michael Campbell by one, with Vijay Singh and England's Gary Evans part of a logjam on three under.
Late-finishing Scot Alastair Forsyth joined the leaders with three holes left but bogeyed his last two holes.
Early leader Ernie Els dropped back after aceing the eighth hole, while Tiger Woods finished at one under.
Casey was delighted to have finally played to his potential in his home major but was certain his 66 could have been even better, despite seven birdies more than wiping out two bogeys.
"Today was a good round, but I still left a few shots out on the course. It could have been very, very low," he said.
"I feel my game has a lot of potential and this is the tip of the iceberg.
"I'm not saying I'm going to win dozens of majors, or anything like that, but I can compete in the Open for the next 15, 20 years.
"I haven't been playing great golf in the last couple of weeks, so it's nice to come out with a strong opening round.
"I did a lot of work on the game this week and it seems to be paying off."
Levet, who won last week's Scottish Open at Loch Lomond to qualify, carded four birdies, including three in a row from the fourth, to reach four under at the turn.
His first drop shot came at the 10th but he birdied the 11th and 17th to go back to the top of the leaderboard. He lost in a play-off to Els at the 2002 Open in Muirfield.
Forsyth, meanwhile, was in the second last group of the day but ignored the evening chill to creep alongside the leaders on the 15th before falling back.
Northern Ireland's Darren Clarke was also on four under playing the 18th but fired his second shot out-of-bounds to slip back to two under.
He joined a group which included local hero Colin Montgomerie, Els and US Open champion Retief Goosen.
Defending champion Ben Curtis struggled to a four-over 75, while Masters winner Phil Mickelson carded a 73.
Spanish star Sergio Garcia also lurched to a 75 as English hopeful Nick Faldo struggled to a 76, as did Ireland's Padraig Harrington.