Among those tied with Thomas was Australian Curtis Luck, the US Amateur winner and runner-up at World Amateurs in Mexico.
Thomas was five behind leader Australian Cameron Davis, who was last year thwarted from winning the title by rain on final day.
Davis had eight birdies in a superb seven-under 65 for a two-shot lead over Yuwa Kosaihara, who has made cuts in his last two starts in pro events on Japan Tour, and fellow Aussie Brett Coletta (67).
Kshitij Naveed Kaul, the youngest in the Indian squad at 16, after finding his first birdie only on the 14th hole, rounded off the day with a second birdie for a 74 that placed him tied 29th.
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The other Indians struggled as Mangalore-born Rigel Fernandes, now at University of South Florida, dropped three shots in last three holes for a 76, while Viraj Madappa, whose golf bag arrived two days after he did from Texas, was undone by a quadruple and shot a similar 76 in tied 50th.
The 17-year-old Thomas produced a great fight back after being three-over through three holes.
"I had seven birdies and I came back with a two-under, so there are mixed feelings. But after the way I started with three bogeys in a row from 10th to 12th, I am very happy with the finish, which had three birdies in last three," said Thomas.
Three bogeys in first three holes threatened to derail Thomas' bid, but a par on par-three 13th stemmed the rot. Then he wiped out the bogeys with a streak of three birdies from 14th to 16th.