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 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."