Seventy-two players completed their first rounds Thursday at Laguna National Golf and Country Club. The remaining players will return Friday morning to finish their first 18 holes.
Paul Broadhurst, Simon Khan and Jeev Milkha Singh share second place at 6- under-par 66.
Ernie Els, who has won twice in the last two weeks on the PGA Tour, is 5- under par through 15 holes as is defending champion Arjun Atwal. Wen Teh Lu, Andrew Marshall, David Park and Gustavo Rojas completed their first rounds at 5-under 67.
Barham, who birdied his second hole, did not look like a potential leader when his 5-wood tee shot at the 13th, his fourth of the day, found water. He carded a double-bogey six but soared up the leaderboard with six birdies in a seven-hole span, starting at the 17th.
The 26-year-old Challenge Tour graduate collected a pair of birdies at seven and eight to go one clear of the field.
Barham only arrived in Singapore on Tuesday from South Africa and limited his practice to walking nine holes that afternoon and walking the other nine on Wednesday.
'Funnily enough, the last time I shot 65 in the first round of a tournament was in the Scottish PGA Championship at Gleneagles in 2000 when I was first reserve and didn't play a practice round then either,' said Barham, who took fourth in that event, his best finish on the European Tour. 'Maybe I should do that all the time, just travel out to a tournament a day later and just walk the course rather than play it - it certainly seems to be working.'
Els set a PGA Tour record for lowest score in relation to par two weeks ago at the Mercedes Championships and followed last week with a thrilling playoff victory over Aaron Baddeley at the Sony Open in Hawaii.
On Thursday, Els was only 1-under par through the first 10 holes but caught fire on the back nine. He birdied four of his next five to get within one of the lead before the horn sounded, stopping play.
'I'm not really disappointed to have stopped,' Els said. 'I can go home now because I'm happy with the way I came back on the back nine. I hit a good drive down the 16th so we can go home happy and come back tomorrow, it doesn't make any difference.'
John Bickerton, Nick Dougherty, David Drysdale, Rick Gibson, Per Nyman and Lian-Wei Zhang finished at 4-under-par 68. Stephen Leaney is four-under through 14, Ted Purdy is minus-4 through 12 holes and Mark Pilkington is 4-under as he makes the turn.