Djokovic, 38, beat Tien, 6-1, 7-6, 6-2 to improve to 19-0 in US Open first-round matches, tying Roger Federer for first all-time. But he labored with low energy, a nasty blister on his right big toe and Tien’s level, especially in the second set.

Novak Djokovic is on a race against time to clinch his 25th Grand Slam title. Since winning his 100th career title at the Geneva Open, the Serbian has only shown up for the Grand Slams.