Roger Federer has beaten Florian Mayer for an eighth time without a loss to knock the defending champion out of Germany's Halle Open in the quarter-finals 6-3 6-4.
Federer, who lost to German veteran Tommy Haas in Stuttgart, has not dropped a set in three matches this week while improving his record in Halle to 57-6.More news: Texas mom arrested after 2 children die in hot car during 'lesson'
The pair held serve for nearly two hours before Zverev managed the first break of the match early in the third set and the 20-year-old broke again two games later to finish the Spaniard off.
The 28-ranked Khachanov fired a backhand long in the breaker to send Federer through to a showdown with Zverev.
"It was perhaps not always very handsome to watch, but it's good to be in the final in the second tournament after my return", the Swiss said afterwards.
Zverev, who had lost his first eight matches against top-10 players then beaten seven of the next 11 - including Federer in the semi-finals here a year ago - encountered the Swiss on one of his magical days. He tried his best to upset Federer's flow.More news: Home sales up in May, but buyers face sharply rising prices
"It was all a little uncertain because I don't know him that well", said Federer, who rued uncharacteristic unforced errors. I'm very happy for him, how he's developed in the last years. "It's a wonderful feeling to win here again because I'm not sure if I'll ever get a chance to win this again so it's important to enjoy this one". "It's warm, it's hot, I'm a little bit exhausted".
"Roger is the best player of all time on grass, it will be a very hard match", said the young German.
"Singles are going to be first, anyhow, so that is not going to make much of a difference", said the world number seven and champion at Queen's in 2012.
Federer, whose first success at Halle came in 2003, has won 92 career titles from 140 finals.More news: Deadly heatwaves will continue to rise, according to study
Kvitova saw off Australia's Ashleigh Barty 4-6, 6-3, 6-2 in only her second tournament since the start of her comeback at the French Open last month.