Nagal entered the tournament through the qualifying rounds, whereas Jung entered as a lucky loser.
Sumit Nagal came from behind to beat Jason Jung of Chinese Taipei 3-6 6-2 6-0 to book his place in the quarterfinals and keep the Indian challenge alive in the singles event of the Chennai Open ATP Challenger tennis tournament in Chennai on Thursday. (More Tennis News)
Nagal, who came through the qualifying rounds, could not break the service of his 33-year-old Taipei opponent, a lucky loser in the opening set.
Jung won a lot of points on his serve and never faced a break point while capturing the first set 6-3.
The 25-year-old Indian raised his level of play and began to return well. He broke his opponent's serve thrice in the second set while losing his own once to level the match.
In the third set, it was Nagal all the way as he landed 75 per cent of his first serves and returned superbly to win the decider without dropping a game.
Nagal, ranked 506 in the ATP rankings, will take on Great Britain's Jay Clarke (313) in the quarterfinals.
Clarke defeated eighth-seed Dimitar Kuzmanov 3-6 6-3 6-4 to advance to the next round.
Meanwhile, no.2 seed James Duckworth beat Serbia's Hamad Medjedovic 6-2 7-6 (3) to book his spot in the last eight.
He was joined in the quarterfinals by compatriot Max Purcell, a 7-5 6-3 winner over Petr Nouza of Czech Republic.
In the doubles, the top-seeded Indian duo of Jeevan Neduncheziyan and N Sriram Balaji beat compatriots Nagal and Sasikumar Mukund 7-6(5) 6-0 to sail into the semifinals.
Results: Men's singles (pre-quarterfinals): Sumit Nagal (India) beat Jason Jung (Chinese Taipei) 3-6 6-2 6-0; James Duckworth (Australia) beat Hamad Medjedovic (Serbia) 6-2 7-6(3); Max Purcell beat Petr Nouza 7-5 6-3; Jay Clarke beat Dimitar Kuzmanov 3-6 6-3 6-4.
Doubles (Quarterfinals): Jeevan Neduncheziyan and N Sriram Balaji beat Sumit Nagal and Sasikumar Mukund 7-6(5) 6-0; Jay Clarke and Arjun Khade beat Ji Sung Nam and Min Kyu Song 1-6 7-6 10-8; Petr Nouza and Andrew Paulson beat Francesco Maestrelli and Luca Nardi 6-3 6-4; Sebastian Ofner and Nino Serdarusic beat Ryan Dalibor Svrcina 6-1 6-7 10-4.