Former world No 1 Djokovic, who has been playing with a sleeve over the troublesome elbow that sidelined him for six months, paid the price for a lack of competitive tennis as he suffered a 7-6 (7-4) 7-5 7-6 (7-3) defeat to relentless 21-year-old Chung.
Meanwhile, unheralded Sandgren added to his earlier victory over 2014 champion Stan Wawrinka by crushing fifth seed Dominic Thiem's hopes in five sets.
