Federer and Zverev came into their clash having won two games apiece against each other. The Swiss world No 2 beat the German in Halle but Zverev returned the favour at the Montreal Masters.

Rafael Nadal's dramatic withdrawal has left Federer as the only genuine star left in London and is the clear favourite to lift his seventh year-end title at the grand old age of 36.

But Zverev is setting tongues wagging after an impressive year in which he has won five tournaments. Federer was forced to repel three break points against the German as early as the opening game. That was the closest any player came to a breakthrough in a contest which was hotting up nicely.

Federer was motoring by the ninth game, which he wrapped up in just 63 seconds, but his opponent was happy to go toe-to-toe, and denied the 19-time Grand Slam champion two sets points to take it into a tiebreak.

The 36-year-old usurped a 4-0 lead to the 6ft 6in bit-hitter, who netted on his own set point opportunity at 6-5, to pay the price as Federer took it 8-6 after 54 minutes of spectacular action.

When Federer broke a frustrated Zverev at the start of the next set, the writing looked to be on the wall. But the third seed composed himself to hit back and complete a remarkable turnaround to take it into a decider following a loose forehand from the Swiss on his third set point chance in the 12th game.

The Swiss star composed himself again to break down a man 16 years younger with six straight games in the third set to make it through to the semi-finals after two hours and 11 minutes of thrilling action.

Shot of the match

Federer enjoyed the battle

"It was a good battle and I always enjoy myself at this beautiful arena," said Federer. "I have so much fun playing in London and I have been so successful here. Thank you for making it such a special night.

"I'm happy and relieved that I can play freely against (Marin) Cilic, instead of it being a nail-biter. It's been a tough group so to be through in two matches is great."

Experts' verdict

Federer raised his game in the decider. He moved his feet and moved better, while Zverev's serve percentage was in the low 40's (44%) which isn't good enough. Don't ever question this man - he is the greatest for a reason

Greg Rusedski

It was a terrific match. Zverev fought tooth and nail but Federer is the greatest at raising his game and intensity levels, which is very difficult to overcome

Peter Fleming

What's next

Zverev faces a crunch clash against Jack Sock on Thursday

Table-topper Federer will play his last round-robin match against Marin Cilic, who he beat in the Wimbledon final earlier this year, while Jack Sock will take on Zverev in a winner-takes-all clash for the right to join the world No 2 in Saturday's semi-finals.