John Isner once predicted big things for Alexander Zverev.

The American No. 1 believes Zverev is poised to pull off a major result in London tomorrow.

Watch: Zverev Sweeps Isner

Isner says Zverev has a "very good shot" of toppling six-time champion Roger Federer in the semifinals and reaching the Nitto ATP Finals championship match.

The 21-year-old Zverev blasted 18 aces, including a 143 mph missile to save a set point, defeating Isner, 7-6 (5), 6-3, to become the youngest year-end semifinalist since a 21-year-old Juan Martin del Potro reached the 2009 final falling to Nikolay Davydenko.

Isner said if Zverev serves with the same authority he has a very good shot of upsetting Federer, who has won three of their five meetings.

"It should be a very good match, I think one that the fans will certainly enjoy," Isner told reporters in London. "If Sascha [Zverev] serves like that, he could have a very good shot at winning. We all know what Roger does so well. He serves incredibly well. It's definitely strength on strength.

"But I do believe, I think it will be a very close match tomorrow between those two. a little bit."

Federer is contesting his 15th semifinal in 16 ATP Finals appearances and Isner says experience gives the 37-year-old Swiss a slight edge.

"You have to say Roger is the favorite," Isner said. "He is ranked higher right now, had a little bit better year, but only slightly. As I said, I think it can be a very, very close match."

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