Sinnott, 22, is renowned in elite amateur circles for his long hitting, but today he played the Composite Course at Royal Melbourne like an expert, deploying two irons off the tee and using his driver just twice.
He also had the good fortune of the benign early morning conditions that favoured all the players on that side of the draw, rolling in seven birdie putts including a 10-footer at the 17th hole and a 20-footer at the 18th to complete an outstanding day.
Sinnott leads the championship by a shot over the 2012 winner, China's Guan Tianlang, who was steady all day in carding a 4-under 68 with five birdies and one bogey.
A group of others are at 3-under: Australia's Antonio Murdaca, Chieh-Po Lee of China and Tawan Phongphun of Thailand.
As the host country, Australia is entitled to 10 players in the 115-player field, and the consensus is that they will be strongly favoured by intimate knowledge of Royal Melbourne; a world class links-style course with super-slick and undulating greens.
Sinnott is a case in point. He lives in the Melbourne suburb of Williamstown, plays his golf at Metropolitan Golf Club, also in the famous Sandbelt just a few miles away, and estimated that he had played Royal Melbourne at least 30 times, including the past four years in an elite amateur tournament, the Master of the Amateurs.
His knowledge led him to take a conservative path, and not to try to tear the course apart. He hit 14 of 15 fairways, and 16 of 18 greens in regulation.
"I don't think you'll ever see Royal Melbourne play as easy as that,'' he said after his round. "In saying that, weather-wise, there's not much wind out there and to be honest, there were a few friendly pins out there.
"It's never an easy golf course in that if you get the ball on the wrong side of the hole, you're scrambling and you're working really hard to make pars, and sometimes you just can't make pars if you're in some spots out there.''
Sinnott is the 66th-ranked amateur in the world, and his recent form is excellent, including a tie for sixth at the Western Australian Open, a professional tournament, in Perth just last week. He was a semi-finalist in the Australian Amateur Championship this year, and was the Victorian boys' champion in 2009.
With a place in the 2015 Masters at Augusta National Golf Club and a qualifying berth for the Open Championship at St Andrews on offer this week, Sinnott said he would focus on the process. "Once I'm out there, it's just trying to keep my ball in position and hit my spots. But it is a nice little prize!''
China's Guan is a stronger and better player than the 14-year-old who created world headlines in winning the Asia-Pacific Amateur Championship two years ago, and today's round showed that fact. Guan said he was flying the golf ball at least 20 yards farther than he could manage two years ago, and he is naturally a more mature 16-year-old.
"I think it (the win in 2012) changed my life a little bit, but I think I'm still doing my own thing, still on the right track,'' he said. "I felt a lot better from the last couple of months going into this tournament, so I think I will give a pretty good performance this week.''
Virtually all of the leading players came from the morning groups, and the southerly breeze stiffened in the afternoon, making life tough for the players who had afternoon draws.
Included among those on the wrong side of the draw were Chinese Taipei's Asian Games champion Pan Cheng-Tsung, who managed a 73, and South Korea's Gunn Yang, the U.S. Amateur Champion, who posted an 81.
Those players will hope to extract a better result tomorrow, when they play in the morning.
Meanwhile it was announced today that The Clearwater Bay Golf & Country Club in Hong Kong will host the 2015 edition of the Asia-Pacific Amateur Championship. Taking part in the announcement, R&A Chief Executive Peter Dawson said Hong Kong had "a very strong history in the game of golf, and we are very much looking forward to going there."
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