Novak Djokovic says he is "trying to solve the riddle" and find his best form after fighting back from a set down to beat American qualifier Zachary Svajda in the US Open second round.

The 24-time Grand Slam champion appeared subdued in the early stages of the match before powering through to a 6-7 (5-7) 6-3 6-3 6-1 win.

"I'm not pleased with my level of tennis, but, you know, you have days like this where you're not playing at your best, but you just kind of find a way," Djokovic said.

"I'm just trying to solve the riddle once I'm on the court."

The 38-year-old said he was "a bit with frustrated with his game" as he continues his bid for a standalone record 25th major trophy.

"It's not like I'm not finding joy on the court competing. I enjoy competing, but I don't enjoy not playing well," the Serb said.

"That's why I put extra pressure on myself and my team to be better the next day, the next match," he said.

"It's not a motivation thing. I kind of go through stuff internally. You don't want to know the details of what I'm going through and telling myself."

Djokovic has already set another record in New York in reaching the third round at a major for the 75th time - one more than Roger Federer's tally.

He will face Britain's Cameron Norrie - who he beat at the French Open earlier this year - in the third round.

Playing in the morning session on Arthur Ashe Stadium - an unfamiliar timeslot for a player accustomed to headlining the tournament's coveted night session - Djokovic seemed out-of-sorts in the bright sunshine.

The seventh seed had looked fatigued in his first-round match and seemed hampered by similar issues early on against home hope Svajda.

The world number 145 went viral before the match after posting a video of his excitement about playing Djokovic but he showed no signs of being overawed, holding his nerve to edge the first set.

But Djokovic, as he so often does, immediately responded, and reeled off 11 of the final 12 games as Svajda struggled with cramp.

Fritz advances as seeds fall

Like Djokovic, last year's runner-up Taylor Fritz also dropped the first set but came through 4-6 7-6 (7-3) 6-2 6-4 against South Africa's Lloyd Harris.

Fourth seed Fritz, the highest-ranked American in the men's singles draw, will now face Jerome Kym after the Swiss qualifier upset 30th seed Brandon Nakashima of the United States.

Fritz was joined in round three by two-time semi-finalist and 17th seed Frances Tiafoe, who beat fellow American Martin Damm 6-4 7-5 6-7 (8-10) 7-5.

Twelfth seed Casper Ruud became the second-highest men's seed to fall so far, losing 6-4 3-6 3-6 6-4 7-5 to Belgium's world number 107 Raphael Collignon.

It completes a miserable year of Grand Slam results for Norwegian Ruud, who also lost in the second round of the Australian and French Open, and missed Wimbledon with a knee injury.

Collignon now meets 20th seed Jiri Lehecka, who progressed with a 3-6 6-0 6-2 6-4 win over Argentina's Tomas Martin Etcheverry.

Another seed to fall was 16th-ranked Jakob Mensik - the 19-year-old, who is the highest-placed teenager in the world rankings, lost 7-6 (7-2) 6-7 (5-7) 6-3 4-6 7-6 (10-7) to Ugo Blanchet of France in a match lasting almost four-and-a-half hours.

It was a good day for other Frenchmen at Flushing Meadows, too, as Arthur Rinderknech upset 18th seed Alejandro Davidovic Fokina 6-4 3-6 2-6 6-2 6-3 in a five-set tussle, to earn a third-round meeting with compatriot Benjamin Bonzi.

Bonzi, who knocked out Daniil Medvedev in an extraordinary first-round marathon, went the distance again, coming from two sets down to beat Marcos Giron of the USA.

Meanwhile, 21st seed Tomas Machac of the Czech Republic saw off rising Brazilian star Joao Fonseca in 7-6 (7-4) 6-2 6-3.