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.

Andrew Castle feels Novak Djokovic's Grand Slam-winning days may be over, with Carlos Alcaraz and Jannik Sinner emerging as dominant forces. The Serb, stuck on 24 majors since the 2023 US Open, faces doubts about adding a 25th title amid younger rivals' rise.

As a tennis veteran, Djokovic will know match sharpness is very different to playing practice sets, so it was no surprise that his physical levels collapsed mid-way through his match against the young and hungry Tien.