Korean qualifier Hyeon Chung produced the first upset of the ATP Chennai Open by ousting last year's finalist Borna Coric with a straight-set win in the opening round of the USD 505730 hard court event in Chennai today.Coric, the world number 48, struggled with his first serve and forehand today while Chung was unrelenting in his 6-3 7-5 win over the Croat. Chung will next challenge Israel's Dudi Sela, who convincingly defeated Bosnia and Herzegovina's Damir Dzhumhur 6-2 6-2.Another qualifier to make the second round was Slovakia's Jozef Kovalik, who sent packing Portugal's Elias Gastao as he rallied to eke out a 6-7(5) 6-4 6-2 win in two hours and 20 minutes. He next books a match with top seed and tournament favourite Marin Cilic, who had got a bye in his opening round.Russia's Daniil Medvedev also made it to the next round with a 6-3 6-3 win over Brazilian Thiago Monteiro. Belgium's Steve Darcis subdued Croatian qualifier Nikola Mektic 6-2 6-3 to set up a second round contest with third seed and world number 27 Albert Ramos-Vinolas from Spain.