Colomo is ranked 507th and does not have a European card at present, but the Asian Tour member’s six-under-par 64 left him one ahead of compatriot Miguel Angel Jimenez and Australian Andrew Dodt.
A tired-looking McIlroy, who last Sunday clinched his European and US Tour money list double, managed only one birdie as he began the defence of a trophy he lifted by holing a bunker shot on the final hole last December.
There was none of that inspired play from the 23-year-old on his return to Fanling.
He had to play out left-handed from the trees on the sixth, made a mess of his approach to the 14th and three-putted from the fringe at the next.
With bogeys on all three holes he finished the day down in a share of 93rd place in a field of 144 before telling reporters he felt “lethargic”.
The 28-year-old Colomo did not even earn a place on the second division Challenge Tour in Europe last season, but came through the Asian qualifying school and has had three top five finishes there.
With his wife acting as caddie, he started with 10 pars in a row before picking up birdies at five of the next six and finishing with another.
“The first six months it was very hard for me and for my wife, but now we are more comfortable and more happy,” he said.
“I think this is very good for us – we feel more strong mentally.”
Jimenez, who will be 49 in January and finds himself in danger of falling outside the world’s top 100, is trying to become the circuit’s oldest-ever winner on the same course where he triumphed in 2005 and 2008.
He would have led on his own but for two closing bogeys.
“I’m putting very well and it’s a pity I missed a fairway on the ninth and then on the last got a big flyer,” said Jimenez. “It’s an old-fashioned course – not very long, but you need to be very precise.”
At 122nd on the Order of Merit Dodt needs to climb three places to secure his card for next season.
“It’s hard not to notice, but you’ve got to play the shot and play the round and what will be will be,” he said. “You can think about it at night, but not during the round – not at all.”
Ryder Cup captain Jose Maria Olazabal partnered Jimenez, one of his assistants in Chicago, and shot 66 for a share of fourth place with China’s Zhang Liang-wei, Swede Fredrik Andersson Hed, Italian Lorenzo Gagli and South Korean YE Yang, Asia’s only major winner in the men’s game.
Paul Casey managed a 70 despite two double bogeys and that was one better than both Padraig Harrington and Welshman Rhys Davies, the player in the hot seat of 119th on the money list.
Malton’s Simon Dyson finished four off the lead with an opening round of 68, while Hull’s Richard Finch needs to improve after starting with a disappointing 74.
n Sheffield’s Neil Cheetham endured a disappointing day at the South African Open at Serengeti.
The 45-year-old shot an opening round of 74, leaving himself 10 shots behind leader Merrick Bremner, whose opening round matched the course record.