Berlin - German tennis legend Boris Becker considers seven-time Formula One world champion Michael Schumacher as one of the three greatest athletes of all time.
The three-time Wimbledon champion said in an interview that Schumacher is a German sporting legend.
"It is unique to be so successful over such a long period of time, particularly in Formula One," Becker said.
Schumacher - who raced in his first Grand Prix 20 years ago on August 25, 1991, at the Belgian Grand Prix in Spa-Francorchamps - returned to the sport two years ago after retiring at the end of the 2006 season.
Becker said that while he could emotionally understand Schumacher's wish to return to racing, he personally did not think it was possible to hold one's ground against youngsters.
"It simply is not possible to face off with a 25-year-old in one's sport as a 40-year-old," Becker said. "As a sportsman, one simply has one's limits, and when one is 30 or 33, one is sometimes simply not quite as good physically. It is then not simply a matter of the car or tyres but also because one is a bit more cautious."
Schumacher has struggled since returning to the sport with the Mercedes team and has not yet achieved a podium place in any of his 30 races.
Becker, who said he never considered returning to active tennis after retiring in 1999, said he believed the Mercedes was not competitive enough.
"Every Formula One expert knows that one cannot win with the wrong car," he said. "If you put a Sebastian Vettel in a Mercedes, even he would not continue to win races. It's just like that."
Becker, who remains the youngest-ever Wimbledon champion, said he did not believe that Schumacher was in danger of tarnishing his image.
"When one is world champion seven times, there is no way one can damage one's reputation. The fact that he made a comeback was, I think, something very positive for the whole of Formula 1 and particularly for Mercedes Benz."
Becker reacted angrily to calls that Schumacher should call it a day: "It is not the place for former Formula One greats to criticize him in public and to virtually force him to retire. That shows no tact and no class.
"I think he himself knows best that he is not racing in optimal conditions at the moment. But some time it will be over, and we should all enjoy the present chance to see him [race] again."
Becker, who also won the US Open and the Australian Open, said he had many memories of Schumacher racing, like the first time he won the world championships or his lengthy dominance of the sport.
"But for me personally, his last race in Sao Paulo was very memorable because not only was I at the circuit but I also got the chance to ask him questions for a German television station," Becker said. "They were the last questions he was asked. He was a bit surprised, and I was a bit surprised. It was simply a very great moment that I got to experience."