Former world number one Boris Becker is not completely sure Roger Federer can be regarded as the best ever to play the sport of tennis.

Before his career renaissance that began last year, Federer was already considered as arguably one of the greatest players of all time.

However, he further cemented his status as the GOAT when he won his first Grand Slam since 2012 at the 2017 Australian Open.

Federer would continue to defy the odds and his age as the 36-year-old would win another six titles, including a record-breaking eighth Wimbledon crown that took his tally to 20 Grand Slams.

To add to his many achievements, the Swiss ace could be on the verge of becoming the oldest world number one in the history of the game if he reaches the semi-final stage of the Rotterdam Open this week.

However, Becker has cast doubts as to whether the 20-time Grand Slam champion can be called the best ever despite admitting that Federer is the most successful player in the game.

"I don't know. You have to say he is the most successful, but how can you decide for sure that he is the best ever," Becker told Tennis365. "Comparing generations is tough, especially in tennis, but he ticks many boxes as the greatest ever.

"He has 19 Grand Slams, the most ever. He has the most weeks at world No.1, which is an impressive record. However, would he have beaten McEnroe when he was at his very best in the 1984 Wimbledon final using a very different type of racquet to the one he has now? I doubt it.

"Would he have beaten Bjorn Borg at his very best when he was dominating at the French Open at Wimbledon? Again, I doubt it, but that is not to belittle Federer's achievements for one moment."

Becker, though, expressed astonishment as to how Federer is still playing and remaining successful as he heads into his late thirties, adding that he believes he will stick around for some time.

"Who would have imagined he would still be the guy to beat when he was 36," Becker added. "He is still very much here and I still believe he will stay around for some time. He has set the bar so very high in his career and everyone looks for the minor crack in his make-up.

"I am amazed by his commitment to the game considering he has four kids, as that was one of the reasons I decided to quit my career."