German top seed Alexander Zverev praised Chinese opponent Buyunchaokete after a grueling opening round match at the Rio Open on Tuesday.

Zverev admitted he was made to work harder than he had hoped in a 7-6(4), 6-4 victory over Buyunchaokete on the outdoor clay of the Jockey Club Brasileiro.

"It wasn't a great match by me but he played some good tennis," the world No. 2 told reporters after a duel that lasted more than two hours in stifling heat and humidity.

Zverev raced to a 4-1 lead in the opening set before 23-year-old Buyunchaokete suddenly found his groove with an array of impressive groundstrokes and a dependable serve.

Buyunchaokete was unable to convert a set point on serve and Zverev capitalized by wresting back control of the match, though he was far below his best.

The Tokyo 2020 Olympic gold medalist served three double faults and put just 60% of his first serves in play while committing 28 unforced errors.

"He made it difficult for me," Zverev said. "Actually it's surprising because you know I actually played really, really well in practice here the whole week, so I hope it's just the first match. I lost focus. I lost my shots and my rhythm a little bit."

In a drama-filled day, local favorite Joao Fonseca was eliminated from the ATP 500 tournament just two days after winning his maiden title.

The 18-year-old made 34 unforced errors as he stumbled to a 6-1, 7-6(4) loss to 60th-ranked Frenchman Alexandre Muller.

"I knew nerves would hit me and that I would have to face up to the moment in front of so many people," said Fonseca, who clinched the APT 250 Argentina Open title on Sunday.

"I tried my best to do that but I wasn't able to overcome [the nerves] and couldn't play my natural game," the Brazilian added.