Harrison has regularly participated in the third highest tournament category, the ITF Future Tour , since 2011 . In his first participation in a tournament of the ATP World Tour 2013 in Atlanta , he won his first match against Alejandro Falla . He then lost to top seed John Isner in three sets. Together with his brother and doubles partner Ryan Harrison he played at the US Open 2012 after they received a wildcard from the tournament management and reached the quarterfinals. In 2014 and 2015 he did not play a match because he had to cure various injuries.
After his hiatus in 2016, he made his debut in a Grand Slam singles tournament at the US Open after battling his way through qualifying. There he lost with only three game wins against Paul-Henri Mathieu . In 2018 he stood for the first time in a final of a challenger , the second highest tournament category, where he was defeated by Hugo Dellien . At Wimbledon he managed to qualify for a Grand Slam for the second time. Here he lost in four sets against Kei Nishikori . In July 2018, he reached 198th place, his previous best value in the world rankings .