The 20-time Grand Slam champion, 37, lost to world number 55 John Millman in the quarter-finals.

Swiss Federer has scaled down his playing commitments, instead building his schedule around specific events.

"Eventually it catches up with you and you get unexplainable losses and bad performances," said Cash.

He told BBC Radio 5 live: "It will happen once in a while and then a little bit more and a little bit more. Then you say either play a little bit more or retire.

"He may or may not make that decision this time. I'm not saying retirement is imminent but he might think enough is enough.

"When you come towards the end of the career these losses just happen and you scratch your head and ask 'what went wrong there?'"

Second seed Federer was targeting a sixth US Open victory, having not won at Flushing Meadows since 2008.

But, after winning the opening set, he produced an erratic display to lose to Australian Millman in four sets.

Federer had never lost previously at the US Open to a player ranked outside the world's top 50.

He skipped the clay-court season for the second successive year, returning to play two grass-court tournaments before losing to Kevin Anderson in the Wimbledon quarter-finals.

That was his earliest exit at the All England Club since losing in the second round to Ukraine's Sergiy Stakhovsky in 2013.

He responded by taking another break before playing one tournament in Cincinnati, where he lost in the final to Novak Djokovic, in the build-up to the US Open.

Federer says he understands being regularly asked about retirement, having faced with this line of questioning for "like nine years".

Last week he insisted he was not ready to give up.

Doubts have been voiced before about his longevity, notably when he went almost five years between winning Slams after his Wimbledon triumph in 2012.

Yet, after taking the second half of the 2016 season off with a knee injury, he has returned to win three more majors and also became the oldest world number one in February this year.

Will Federer win another Grand Slam title?

BBC Sport tennis correspondent Russell Fuller

Let's not forget that Federer will remain the world number two at the conclusion of these championships, but the match conjured up memories of his fourth-round defeat by Tommy Robredo in New York in 2013.

Then, Federer had back problems and was five years younger. I thought he was unlikely to win another Grand Slam and I am so delighted he proved me wrong three and a half years later with his astonishing victory at the 2017 Australian Open, which paved the way to two further Slam triumphs.

I do not want to make the same mistake again. When he is refreshed at the start of the year in Melbourne, or on the grass of SW19, who knows?

But bear in mind that Ken Rosewall is the only man in the Open era to have won a Grand Slam after celebrating his 37th birthday.

In this phenomenal era, and over five sets in the sapping humidity of New York, we should not be at all surprised Federer has fallen short, but we are allowed to feel a little glum.