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- Dubai 05:26 06:45 12:11 15:10 17:32 18:50
The former paratrooper continued the rich vein of form he showed last year when he won the Asian Tour Order of Merit title for an unprecedented third time to get to eight under for the tournament.
That was one clear of a trio comprising European No.1 Lee Westwood, who had a tournament-best 65, Spanish veteran Miguel Angel Jimenez, who had a 67, and Welsh journeyman Stephen Dodd, who had a 69.
Defending champion Rory McIlroy of Northern Ireland was on six under after a 70, tied with big-hitting Spaniard Alvaro Quiros (69), Marcus Fraser of Australia (68), Edoardo Molinari of Italy (70) and Charl Schwartzel of South Africa (70).
Thongchai's bogey-free round saw him reach the turn in three-under 32, and then he added three birdies in five holes from the 13th to edge out into the lead.
He put his improvement in form over the last year down to a strict fitness regime that has seen him overcome severe back problems that laid him low two years ago and to his improved putting.
Thonhchai, who turned 40 last November, said: "It was very hard with my back, a break of eight weeks and we did a lot of stretching and then a lot of strengthening.
"And then last year, after half the year, my back is getting better and then I played quite well for the last six months until the end of the year and started again these last three weeks."
It was a consumate performance from the 36-year-old Westwood, who clinched the inaugural Race To Dubai title last November by winning the season-finale at the nearby Earth Course.
Like Thongchai, his card was bogey free with three birdies on the front nine and four on the back nine.
His showing was all the more encouraging in that Westwood is a self-styled slow starter to the season.
"If you look historically at my career, I've not started fast in any year," said the world No.4, who could move up a spot if he wins here.
"I do take some time warming up and yesterday I struggled a bit. I'm not a morning person and I was a bit stiff.
"But today it had worn off and I started to loosen up a and felt my swing a bit more."
McIlroy, who was in a six-way tie for the lead overnight, produced a flawless round of golf, with four birdies in a row round the turn, but came to grief at the last when his approach went in the water for a double bogey.
"Apart from that mistake at the last it was another good day's work," McIlroy said.
"Made a few birdies round the turn and could have made a few more. But six-under par isn't too bad going into the weekend."
The Irish prodigy is bidding to make history here by becoming the first man to defend the Desert Classic title in the 21-year long history of the Gulf's premier tournament.
His win here last year was the first of his fledgling career and though he has yet to win another title, McIlroy is now firmly ensconced in the world top 10 as the youngest player in the elite group.
The 46-year-old Jimenez, who changed caddies after two poor weeks in Abu Dhabi and Qatar, had seven birdies against two bogeys to signal his return to top form.
"I hit it very solid and managed to hole a few putts," he said.
There was a fine performance also from US golf great Tom Watson who was making his debut in Dubai at the ripe old age of 60.
Last year's British Open runner-up shot a fine two-under 70 and at one under for the tournament comfortably made the cut which was more than could be said for Europe's Ryder Cup captain Colin Montgomerie who struggled to a 76.
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