After dropping just two games in his second-round opener against Czech qualifier Dalibor Svrcina, Sinner was caught napping to drop his serve in the opening game.

He responded with confidence against a potentially tricky opponent known for producing winners.

Sinner quickly recouped the break and grabbed another in the eighth game.

"It's a very tough second match for me," the Italian said. "He's a very high-quality player... he had great wins in the past so I'm very happy about today's performance."

With one set in his pocket Sinner was in full control in the second, opening with a break on the way to a 5-2 lead and closing it out with a love game.

"I tried to stay solid on the baseline but also a bit more aggressive in the second set," Sinner said. "It was a tough start for me with a break down but I'm happy how I reacted."

Sinner faces the 19-year-old Fonseca for a place in the quarter-finals.

Fonseca, who saved two match points in his second-round victory over Karen Khachanov, beat another seeded player, American Tommy Paul, 6-2, 6-3.

Fonseca brought the heat, displaying thunderous groundstrokes as he reached the fourth round of a Masters 1000 for the first time.

He said he was eager for the chance to test himself against four-time major winner Sinner.

"We tennis players, we look to playing against top players so it's going to be a pleasure -- and hopefully I can get the win," he said.

Fourth-seeded German Alexander Zverev held off a determined Brandon Nakashima 7-6 (7/2), 5-7, 6-4.

Zverev didn't have a break point against the 30th-ranked American until the third set. He missed his first two opportunities before finally making one count, breaking Nakashima in the final game to seal the victory.

"He played a fantastic match today," Zverev said of Nakashima, who had 17 aces to Zverev's 15.

"He served like I have never seen before. He was hitting his spots well. Every ace was more or less on the line for him, so I've got to give credit to him. He played a great match."

Zverev gained the upper hand in a close-run contest with a dominant first-set tiebreaker, but the American converted his fifth break chance for a 6-5 lead in the second and leveled the match with a love game.

"Definitely happy to get the win because that was a difficult match," said Zverev, who next plays American Frances Tiafoe.

- Tien tops Shelton -

Frances Tiafoe beat Flavio Cobolli 6-1, 6-2 in a rematch of last week's final in Acapulco won by the Italian.

Rising US talent Learner Tien knocked off eighth-seeded compatriot Ben Shelton 7-6 (7/3), 4-6, 6-3.

Tien peppered big-serving Shelton with 15 aces while Shelton, apparently still feeling the effects of the illness that slowed him in a second-round win over Reilly Opelka, produced just eight.

"It's great," Tien said of lining up a last-16 shot at 18th-seeded Spaniard Alejandro Davidovich Fokina -- a 6-2, 4-6, 6-2 winner over 12th seed Jakub Mensik.

But he knew Shelton wasn't at his best.

"I don't think he's been feeling great all week, but he's an amazing competitor and he came out and gave it his all," Tien said. "So I just want to say, huge props to him."