Now the question is whether he can hang on to bag the biggest win of his young career come Thursday.
Just three shots back are Luke Clanton, who won the North & South Amateur at Pinehurst in July, while a pair of Virginia Cavaliers, George Duangmanee and Deven Patel, are four off the pace at 11-under 201.
The golfers took advantage of soft conditions at Miami Beach Golf Club where heavy overnight put the lift, clean and place rule in action for Wednesday's third round. The result was a round where the scoring average went from 72.6 Tuesday to 70.1, which is a massive drop.
After carding rounds of 64 and 65, Guan turned in a third-round 67 on Wednesday. He birdied three of his first four holes out of the gates and after giving a stroke back on the sixth hole, finished his outward nine with a birdie to turn in 31. A double bogey on the par 5 10th was followed by birdies on 11, 12, and 17 for a 4-under round of 67.
Clanton fired a 6-under 65 that left the Florida State freshman just three off the lead heading into tomorrow's final round. The Hialeah, Fla. native made four birdies in his first nine holes and three more on the back offset his only bogey of the day on the 16th.
University of Florida's Luke Poulter and Purdue's Nels Surtani fired the low rounds of the day with a pair of 64s.
Should Guan hang on, he would become the youngest winner of the South Beach International Amateur by three years. He certainly has the pedigree. Currently ranked 14th in the AJGA rankings, Guan was the youngest player to qualify for the U.S. Team in the Junior Presidents Cup held last September at Quail Hollow. He also had a breakthrough win at the Wyndham Invitational last June.
Guan, Clanton and Patel will be paired together for Thursday's final round.