David Goffin has labelled Roger Federer "unbelievable" after hailing the Swiss ace's continued success despite being in the latter stages of his career.

The Belgian admitted that he is not surprised by the quality of tennis being produced by the 36-year-old, but more by his ability to sustain himself physically despite playing at a high level on a weekly basis on the ATP Tour.

Questions were raised about Federer's future in the game when he aggravated a knee injury in 2016 and had to spend six months on the sidelines. The Swiss tennis legend had failed to win a Grand Slam title since 2012 before his return from injury in 2017.

However, he silenced his doubters by capturing the 2017 Australian Open title; he followed it up with six more titles including a record eighth Wimbledon title. Federer ended the season ranked number two in the world.

Federer made it 20 major titles by defending his crown in Melbourne earlier this year and has now set his sights on overhauling Rafael Nadal for the top spot in the ATP men's singles ranking. The Swiss star is currently playing at the Rotterdam Open and will become the top ranked player if he makes the semi-final — he is currently preparing to play Philipp Kohlschreiber in the second round.

The 20-time Grand Slam champion has played a limited schedule since his return from injury in 2017. Federer has followed a fitness first mantra, which saw him miss the entire clay court swing of the season in order to remain in peak physical condition for the grass and hard court events. He is likely to follow the same strategy in 2018.

"It's unbelievable. He is still winning, even more than before. I'm not surprised about his tennis because he has everything in his racket since the beginning of his career," Goffin said, as quoted by the Express.

"But I am surprised how physically he can maintain that level. How he can play tournament after tournament and stay the same level physically so he is really strong.

"He knows himself really well so knows when he has to rest, when he has to play, when he's 100 per cent fit. So he is really impressive," the Belgian explained.

Goffin, who was the only player to beat both Federer and Nadal in 2017, is also playing the Rotterdam Open and has progressed to the quarter-finals where he will face Tomas Berdych. However, a potential clash with Federer will only ensue if both players make it to the finals.

Roger Federer
David Goffin was the only player to beat both Nadal and Federer in 2017 Getty