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 fears his lack of physical conditioning may come back to haunt him as he chases a record 25th Grand Slam singles title at the US Open.