It is the second straight time the Italian has qualified for an ATP 1000 quarter-final and he will face a stern test in second seed Alexander Zverev.
Zverev won a gold medal in the men's singles event, which has managed to elude those two superstars in an individual capacity.
He started his professional tennis career in 2016. He won his first ITF Grade 1 in Canada at the Repentigny Internationaux de Tennis Junior
Zverev did make an outstanding comeback in the second set after losing the first, but his opponent on the day was just too good.
The 20-year-old Italian erased five match points against Pablo Carreno Busta in a 5-7, 7-5, 7-5 victory.
Tiafoe made his pro debut in 2015, finishing the year as world No. 180 and the youngest guy in the ATP Top 200 rankings.
Harris has three ATP Challenger singles and two Challenger doubles trophies, as well as 13 ITF singles and four ITF doubles titles.
Alcaraz has won three ITF Men's World Tennis Tour titles and four ATP Challenger Tour titles since turning professional in 2018.
Thiem is the second-highest ranked Austrian player in history, behind former world No. 1 Thomas Muster and has 17 ATP Tour singles titles.
Matteo Berrettini became the first Italian man to reach a grand slam final since Adriano Panatta won the 1976 Roland Garros title.