Roger Federer's evergreen nature is "beyond belief" according to tennis legend John McEnroe, who believes the Swiss star is playing better at 36 than he was aged 28.

Federer, fresh off the back of holding onto his Australian Open crown last month, is preparing to face world number 116 Ruben Bemelmans in the last 32 of the Rotterdam Open on Wednesday (14 February).

Victory for the eight-time Wimbledon champion in the Netherlands would rival some of his previous career achievements and would see him reclaim the world number one ranking at the expense of Rafael Nadal, who is scheduled to play in Acapulco in a couple of weeks.

Federer will become the oldest No.1 in the history of the sport if he manages to reach the semi-finals in Rotterdam, eclipsing Andre Agassi's record by three years. Such a feat is almost inconceivable in the eyes of seven-time Grand Slam champion McEnroe, who believes Federer is performing better during the twilight of his career than he was in his late 20's.

"It's still beyond belief and one of the craziest things I've ever seen in the 25 years I've been doing commentary and the additional 15 years that I've played," McEnroe said, per SportStarLive.

"To see a guy at that age and play at that level and move better than he did when he was 28, maybe not when he was 25 or 26, but I don't understand how the hell he's doing it. But it's truly amazing to watch."

McEnroe cannot come up with an explanation as to why Federer is still performing at such a high level as he enters his late 30's, but the 20-time Grand Slam champion thinks it is his lighter schedule that is allowing him to prolong his illustrious playing career.

Federer will embark on his third tournament of 2018 when he faces Belgian Bemelmans at Rotterdam on Wednesday, but a busy schedule may now see him put his feet up as the year moves from hard courts to clay.

"The clay season is up in the air. If I am playing a lot during this part of the season it would be hard to play a robust clay season. It would be a light [schedule] if I do play," Federer said, AFP report.

"We will see how it goes. I am healthy and glad to be playing. I should be playing when I feel that way and not play when I don't feel at 100 percent or don't feel ready to do it. That is my secret for me moving forward."