John McEnroe is stunned by Roger Federer's form at the age of 36, describing the Swiss star's latest performances as one of the craziest things he has ever seen.

Federer can become the oldest man to hold the world number one ranking if he reaches the Rotterdam Open semi-finals.

Incredibly, he won his 20th grand slam at the Australian Open last month and has claimed three of the past four majors he has played.

McEnroe, a seven-time grand slam champion, said Federer's form late in his career was amazing.

"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," he said.

"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."

Federer will face Ruben Bemelmans in the first round in Rotterdam.