Seventh seed Stefanos Tsitsipas suffered a shock defeat by qualifier Dominic Stricker in the second round of the US Open.

Tsitsipas of Greece was serving for the match in the fourth set but his Swiss opponent fought back to force a third tie-break and then won the fifth set.

Meanwhile, Novak Djokovic beat Bernabe Zapata Miralles in straight sets.

The Serb, who will return to top spot in the world rankings regardless of how he performs at Flushing Meadows, defeated Bernabe Zapata Miralles of Spain 6-4 6-1 6-1.

Djokovic will next face compatriot Laslo Djere on Friday for a place in the last 16.

"I still feel like I can play better but I'm happy to be back playing in New York," Djokovic said. "That's what gives me strength at 36 years of age. I still have the hunger and the desire to play my best tennis on this court.

"Every single match I play myself the most, in my mind. If I win this battle, then I win the battle outside against the opponent."

Singing Stricker shows confidence in big upset

Stricker is making his US Open debut but the 21-year-old has displayed a calmness and self belief that belies his age so far at this tournament.

Both of those qualities were evident at the changeover in the fifth set against Tsitsipas when, at 5-2 up and a game away from the biggest win of his career, he was singing along to Whitney Houston's I Want to Dance With Somebody as it played out around the court.

"I felt pretty good from the first set onwards," said Stricker, who also beat world number 41 Alexei Popyrin in the first round.

"It was a tough battle but I'm just super happy right now.

"I think right now the belief is very, very high. The confidence is high and I believe a lot in myself."

It continues what has been an impressive year for Stricker. The former junior Grand Slam winner claimed his first major main-draw victory at Wimbledon earlier this summer, beating Australia's Popyrin in the first round before repeating that feat at Flushing Meadows earlier this week.

But it was another early exit for two-time Grand Slam finalist Tsitsipas, who lost to American world number 30 Christopher Eubanks in the fourth round of Wimbledon.

He has also never progressed to the second week of the US Open and last year lost to Colombia's Daniel Galan in the first round.

Tsitsipas served for the match at 5-3 in the fourth set but could not close out and paid the price.

"Usually I'm much tougher mentally and I have shown it by coming back," said the 25-year-old, who has replaced his father Apostolos as his main coach with former Wimbledon finalist Mark Philippoussis.

"Even though sometimes I get broken serving for the match, let's say, I still find ways in the tiebreaker or perhaps in the fifth set if that has ever happened.

"Today was not the case. I was not able to bounce back after it."