The unseeded Chung, 20, went 3-1 up against the 14-time Grand Slam champion in the first set on the newly renamed Rafa Nadal Court at the Real Tennis Club and forced a tie-break, which the Spaniard eventually won with little trouble.
The newly crowned Monte Carlo Masters champion made light work of the second set, rounding off a 7-6(1) 6-2 victory in the Barcelona sunshine with an ace to set up a semi-final on Saturday against Argentine Horacio Zeballos, who beat Russia’s Karen Khachanov 6-4 6-1.
“I have to be honest, I had hardly seen him play, all I had to go on were videos which I watched yesterday,” Nadal said of the little-known Chung.
“These young players have a lot of character so I took to the court with respect, perhaps too much, but in the second set I started dominating.”
Nadal has won every semi-final he has played in the Barcelona Open, where he is chasing a 10th title.
World number one Andy Murray also made the semi-finals but had to dig deep to overcome Albert Ramos-Vinolas 2-6 6-4 7-6(4), avenging his defeat by the Spaniard in the Monte Carlo Masters last week.
Briton Murray was overpowered by Ramos-Vinolas in the first set and had to save three break points in the ninth game of the second before tying the match. He then lost his first service game in the third set.
Ramos-Vinolas, who took a few minutes’ rest in the final set to shake off muscle discomfort, was again unable to seize the chance to see off the number one seed, who came out stronger to win the tiebreak.
“It was obviously a very tough match. It was kind of the opposite of the match we had last week, where probably today he deserved to win, he created a lot more chances, he served for the match and couldn’t quite get it,” Murray told reporters.
“Last week I had 4-0, I felt like I had all of the chances and sometimes on clay matches happen this way so I’m very happy to get through.
“I started coming to the net more and volleying, I felt like I started to dictate more of the points and that was a big difference.”
Murray will play Dominic Thiem on Saturday after the Austrian booked his place in the last four earlier on Friday by easing past Yuichi Sugita of Japan 6-1 6-2.
(Editing by Ken Ferris, John Stonestreet and Clare Fallon)