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.

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. He surprised everyone at Roland Garros and Wimbledon with his dominating performance,