Novak Djokovic improved to 19-0 in the first round of the U.S. Open, battling through some leg troubles to beat Learner Tien 6-1, 7-6 (3), 6-2 on Sunday night.

Novak Djokovic delivered a lesson in Grand Slam tenacity to Learner Tien in the first round of the U.S. Open on Sunday as he battled past the American teenager 6-1, 7-6 (7-3), 6-2 to launch his latest quest for a record 25th major title.

Djokovic largely cruised to a 6-1, 7-6, 6-2 win over American Learner Tien Sunday night to advance to the second round. But he struggled mid-match as Tien pushed him to a second-set tie break that Djokovic won, 7-3.