Novak Djokovic was below his best as he was handed his first genuine test of this year's US Open, overcoming Roberto Bautista Agut in four sets.

The world number one appeared in some trouble at Arthur Ashe Stadium before completing a 6-3 4-6 6-4 6-3 victory to reach the quarter-finals.

Albeit unconvincing, Djokovic's win means he has now reached at least the last eight at 26 consecutive grand slams.

The Serbian never reached the lofty standards he sets as the aggressive and powerful Bautista Agut enjoyed his share of success in the baseline exchanges.

But Djokovic, who finished with 37 unforced errors, would prove classy enough often enough as he set up a meeting with another Spaniard, Feliciano Lopez.

Djokovic holds a 5-0 win-loss record over Lopez, dropping a set just once to the left-hander.

He is also unbeaten against Bautista Agut, who started solidly enough before being broken in the eighth game of the first set.

It looked like it would be another straight-sets win for Djokovic when he broke for 2-1 in the second, only for the momentum to swing.

From a break down at 4-2, Bautista Agut won four straight games to level the match.

On top, Bautista Agut had the nine-time grand slam winner under pressure early in the third set, only to give up a break in the fifth game.

Djokovic lacked his usual energy, as well as focus and killer instinct, as he coughed up a break lead in the fourth.

But he would show a release of emotion when he broke for 3-2 with a drop-volley winner, letting out a roar.

Djokovic broke to love for 5-2 and while he failed to served it out, he took Bautista Agut's serve once more to advance.