#5 Borna Coric
22-year old Borna Coric has already beaten 'GOAT' Roger Federer twice this year, most notably at the semi-finals of the Shanghai Masters.
The promising Croat received the ATP Star of Tomorrow Award in 2014 and started to rise up the ATP rankings rapidly. In October of that year, he defeated Rafael Nadal at the Swiss Indoors lodging the biggest win of his career at that point. He is the only man to have defeated Rafael Nadal twice as a teenager. At the 2017 Madrid Open, he beat then World Number 1 Andy Murray, making it the first time that he had beaten the top-ranked ATP player.
2018 was Coric's breakthrough year. He made the semis of the Indian Wells ATP 1000 tournament where he lost to Federer despite leading by a set and a break. At the Halle Open, he beat Alexander Zverev and Federer to win his first grass court title. He helped Croatia beat France in the Davis Cup final beating Jeremy Chardy in the singles rubber.