The Swiss star, winner of 17 Grand Slam titles, came into the match off the back of a comfortable win over Britain’s Dan Evans on Monday (January 2) and was expected to continue his comeback with another success.

However, Zverev had other ideas producing a superb display to stun Federer 7-6, 6-7, 7-6 in a closely-fought encounter.

“To beat Roger, you really have to play an unbelievable match, from the first point to the last,” the 19-year-old, who beat Federer in the semi-finals of the Halle Open in Germany last June, sad.

“I thought we both played really well.

“I always play unbelievably against him.

“It's always nerve-racking to play against him.

“I always play in front of a big crowd which helps me a lot as well.”

The shock triumph for Zverev gave Germany a 1-0 lead in the Group A tie at the tournament, which sees one male and one female player compete for their nation, but Belinda Bencic squared the contest when she overcame Andrea Petkovic 6-3, 6-4.

Bencic’s win sent the tie into a deciding mixed doubles encounter, played in the Fast4 mixed doubles format, where sets are determined on a first-to-four-games basis.

Federer and Bencic teamed up to beat their German counterparts 4-1, 4-2 to increase their chances of reaching the final.

In the other Group A contest to take place today, France swept aside Great Britain 3-0 to keep their final hopes alive.

World number 42 Kristina Mladenovic battled to a 6-4, 5-7, 6-3 victory over Heather Watson before Richard Gasquet wrapped up the win for France with a 6-4, 6-2 triumph against Dan Evans.

Mladenovic and Gasquet then won the mixed doubles 4-3, 4-3.