"You cannot be serious." Many from the tennis world have been repeating these eternal words, which were first made famous by John McEnroe, since Thomas Muster returned to the ATP Tour last month - at the age of 43.
Many tennis players consider their retirement from the seniors' tour at that age, but the former world No 1, 1995 French Open winner and 1997 Dubai champion is hitting the reverse gear, 11 years after his first-round exit at Roland Garros in 1999.
Muster had never officially retired and has now set his sights on a full return to the ATP Tour with plans to play in around 20 tournaments.
"At the time [in 1999] all I said was I was going on holiday," he said in June. "So now I'm back from my holiday.
"I hated tennis when I stopped in 1999. Now I love it. And, as my body feels good, I want to play as much as possible."
Most people would scoff at that. Muster has played six Challenger tournaments since his return and made first-round exits in five of them; in the other, he lost in the second round.
A wild card at the Austrian Open last month gave him his first ATP Tour game since the 1999 French Open and he lost in straight sets to compatriot Andreas Haider-Maurer, an opponent who has never reached the top 150 of men's tennis.
Yet, there would be many who would fancy Muster's chances against opponents half his age. Determination has always been the hallmark of his career and staging comebacks from impossible situations is not new to him.
In 1989, just after reaching the final of the Lipton Championship in Florida, Muster's career was almost brought to an end by a drunk driver. He was unloading his tennis gear from his car when the other driver's vehicle hit his.
The impact severed both the medial collateral and the anterior cruciate ligament in his left knee, and most people thought Muster, 21 then, would never play again.
Six months later, though, he was back on the courts. Even during the break, he could not stay away from the court and would spend hours hitting forehands, sitting on a specially designed chair.
Remembering that moment, Ronald Leitgeb, his then manager-coach, said: "Two weeks before, when he came out of the hospital, he'd said, 'Ah, I'm not going to run again in my life. I've had enough of physical and athletic training. That's it.' He had such sad eyes. But then he was hitting in that chair, and I could see the fire in his eyes. From that moment I believed he was going to do it." If Muster can find that fire again, Father Time might just relent. He did for the 40-year-old Kimiko Date Krumm this year; Arthur Gore won the 1909 Wimbledon at the age of 41; Ken Rosewall was winning titles and reaching finals in his 40s; Richard "Pancho" Gonzalez won the title in Des Moines three months before his 44th; Martina Navratilova won the Wimbledon and Australian Open mixed doubles titles at the age of 46.