Top-10 stars Alexander Zverev and Matteo Berrettini both clocked solid victories on Tuesday as they ramped up preparations for the Australian Open by keeping Germany and Italy on track at the ATP Cup.

German world number three Zverev came through a workmanlike 6-4, 6-4 clash against American Taylor Fritz, his second straight win of the new season.

Seventh-ranked Italian Berrettini, meanwhile, bounced back from a shock defeat to Australia's Alex Di Minaur on Sunday to battle past Frenchman Ugo Humbert 6-4, 7-6 (8/6), who upset US Open champion Daniil Medvedev at the weekend.

"In general, it was at a good level for the second match of the year, a high intensity and hard-hitting," said Zverev. "Most important is that Team Germany won."

Earlier Jan-Lennard Struff gave Germany a 1-0 lead with a gutsy 7-6 (8/7), 4-6, 7-5 win over big-serving John Isner, who smashed 34 aces.

"It's always tough against John," Struff said. "I had played him four times before and lost four times. So I'm very happy to get the win for my team today."

The win kept Germany's ATP Cup hopes alive after they lost to Britain in their opener, with only the top nation from each of the four groups progressing to the knockouts.

Last year's runner-up Italy suffered a disappointing defeat on Sunday against Australia, largely down to Berrettini losing in straight sets to Di Minaur.

But he made amends against Humbert, winning 85 percent of his first-serve points.

"A really tough match, especially in these conditions. He's a great server, a tricky player, a lefty," said the Italian, who dominated with his serve, sending down 18 aces.

"It's a big weapon that I have and when it is working like this, it helps me a lot."

World number 10 Jannik Sinner set Italy on their way, defeating France's Arthur Rinderknech 6-3, 7-6 (7/3) in the opening singles, rallying from a break down in the second set.

"It's never easy playing against him, it was the third time already. The first set I was under control. In the second set I dropped a little bit of intensity," said Sinner.

Rinderknech defeated Sinner on clay in Lyon last year, and appeared primed to push for another upset, but one error allowed the Italian back on serve and he made no mistake from there.

France is now 0-2 at the tournament, having also lost against defending champions Russia.

Russia and Australia, who both won their opening matches, meet in the evening session with Medvedev playing Di Minaur.

Canada, spearheaded by Felix Auger-Aliassime and Denis Shapovalov, faces Britain in the other night tie.

The Canadians fell to the United States in their opener while the Cameron Norrie-led British upset Zverev's Germany.