My answer will be very US centric because I raced as a top age grouper with a lot of these guys and then/now as a pro with them. So, they are 'older' and may be some new folks that I don't know about. And, these are all males. Laura (Reback) Bennett, Barb, and SheilaT were all short course women who came from a single sport background and had very quick success.
I turned pro in 2004 after finishing 2nd at USAT Amateur Nationals. 2004-2009 I flirted with the top-5 and was regular in the top-10.
TJ Tollakson turned pro maybe a year or so later. He, I believe, had a similar trajectory though he focused more on Long Course racing early and has won both 70.3s and had a few IM podiums, and I believe 1 win.
Michael Lovato has won a few IMs, a few top-10s in Kona, and I believe that his first IM win came not too long after he turned pro.
Tim DeBoom won age group Nationals and turned pro. He had pretty quick success, but I think dabbled in some of the early draft legal racing and short course before moving to long course.
Andy Potts I believe did a race or two after college turned pro, and then made the Olympic Team or was very close to making it. He's had the quickest rise in recent times to my memory.
Hunter came up through the age group ranks.
But, has there been anyone who came from another sport at the National/Olympic level, turned pro in triathlon and in that same year or 1 year later was in the top-5 in relatively solid races? No one that i can think of off the top of my head.
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