Lots of players turn "pro", that is they gain some ATP points here or there. American players to note that played in college (outside of Isner and Johnson), top 500. Players can declare themselves as pro's, and there are a bunch in the 500-1000 range....But these are the Americans I noted at least within the top 500....Sorry if I missed one,
Rank Player Year turned Pro School
62 Tennys Sandgren 2011 Tennessee
84 Mackenzie Macdonald 2016 UCLA
129 Noah Rubin 2015 Wake
189 Chris Eubanks 2017 Georgia Tech
238 JC Aragone 2017 Virginia
251 Mitchell Krueger 2012 Texas A&M
276 Evan King 2013 Michigan
286 Kevin King 2013 Georgia Tech
294 Dennis Novikov 2013 UCLA
298 Collin Altamirano 2017 Virginia
309 Alex Sarkissian 2014 Pepperdine
318 Thai-Son Kwiatkowski 2017 Virginia
360 Maros Giron 2014 UCLA
378 Sekou Bangoura 2011 Florida
436 JJ Wolf Still in College Ohio State
461 Ronnie Schneider 2017 UNC
465 Jared Hiltzik not declared a pro Illinois
469 Alexander Ritschard not declared a pro Virginia
outside the top 500, many are within the last couple of years for instance:
Ryan Shane Virginia
Strong Kirchheimer NW
Tom Fawcett Stanford
Patrick Kypson Texas A&M