Roger Federer needs to play more tournaments following his shock US Open exit or consider retirement, says former Wimbledon champion Pat Cash.

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

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?'”

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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.

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Former champion Juan Martin Del Potro reached his second successive US Open semi-final with a hard-earned 6-7 (5-7) 6-3 7-6 (7-4) 6-2 win over John Isner.

World number 11 Isner took the opening set on a tie-break before the world number three levelled following a break in the fourth game of the second set.

The Argentine won the next tie-break and then broke in the second game of the fourth set en route to victory.

The 2009 champion will face Rafael Nadal or Dominic Thiem on Friday.

More to follow.

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Roger Federer said he was “happy the match was over” after playing in “uncomfortable” conditions during his shock fourth round US Open defeat.

The 37-year-old Swiss – a five-time champion at Flushing Meadows – was beaten 3-6 7-5 7-6 (9-7) 7-6 (7-3) by world number 55 John Millman.

Their match finished just before 01:00 local time but temperatures still hit 30C with high humidity levels.

“I felt I couldn’t get air. There was no circulation at all,” Federer said.

“It’s uncomfortable. Clearly you just keep on sweating more and more and more and more as the match goes on. You lose energy as it goes by.

“At some point also I was just happy that the match was over, I guess.”

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Tournament organisers kept the roof – which was completed in 2016 – on the Arthur Ashe Stadium open on Monday, but world number two Federer believes its installation has reduced air circulation on the court.

“I think just that makes it a totally different US Open,” said the 20-time Grand Slam champion.

“Plus conditions maybe were playing slower this year on top of it. You have soaking wet pants, soaking wet everything. The balls are in there, too. You try to play. Everything gets slower as you try to hit winners.”

Federer’s defeat to Australian Millman marked the first time he has lost to a player ranked outside of the top 50 at the US Open.

Brisbane-born Millman said he “felt like a deer in the headlights to begin with” and had also struggled in the heat, calling the tournament conditions “pretty brutal”.

“I’m not usually much of a sweater. I mean, I was dripping,” said the 29-year-old. “The conditions for the majority of this tournament have been pretty brutal.

“I found it pretty tough early on to hold onto the racquet. It’s kind of moving around in your hands.”

‘Roger is my hero’

Millman – the only unseeded player left in the last eight of the men’s draw – will play two-time champion Novak Djokovic in his first Grand Slam quarter-final on Wednesday.

He admitted to feeling “a bit guilty” after beating Federer, who he described as his “hero”.

“I always was of the opinion that I was in the fourth round for a reason. I’ve never played anyone’s reputation,” said Millman.

“I felt a little bit guilty today because he didn’t have his best day, and that’s for sure.

“I have so much respect for him, what he’s done for the game.”

Millman lost in straight sets to Djokovic at Queen’s in June, but said he has “the belief” he can defeat the Serb in New York on Wednesday.

“I’ll have to improve a lot on the last time I played him. He’s an incredible player. I think he’s in some really good form right now, too,” he said.

“But why not? I think it’s a disservice to who I am if I go out there and don’t have that belief.”

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Andy Murray says he did not want to “let his country down” after deciding not to play Great Britain’s Davis Cup tie in Glasgow next week.

The 31-year-old Scot will instead continue his rehabilitation following hip surgery in January.

With the Davis Cup format changing, it could have been his last chance to play competitively in Scotland.

“I found this decision emotionally challenging,” the former world number one posted on social media.

“Having been born in Glasgow and growing up in Scotland I would never have imagined I would see such passionate fans packing out stadiums for tennis matches.”

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Murray, who helped Britain win the Davis Cup for the first time in 79 years in 2015, has only played seven tournaments since returning to competitive action in June, with his Grand Slam comeback ending in the second round of the US Open last week.

The three-time major champion says he spoke to British captain Leon Smith about only playing in the doubles before deciding his best long-term option was to miss the World Group play-off against Uzbekistan.

Instead, he will continue with rehabilitation work away from the court.

“Having been recommended to take a couple of weeks off hitting to continue my reconditioning, I didn’t want to just show up not ready to perform to a high enough standard and ultimately let my team-mates and country down,” Murray said.

“With this possibly being my last chance to compete in Scotland as a professional I really wanted to be there with the team.”

Kyle Edmund, the British number one, also misses out on next week’s tie, but Dan Evans, Cameron Norrie and Jay Clarke are included.

Jamie Murray and Dom Inglot, specialist doubles players, have also been picked for the World Group play-off tie, which starts on 14 September.

A first-round loss to Spain in February means Great Britain are featuring in the play-offs for the first time since joining the top-level World Group in 2014.

The format of the tournament is changing next year – it is turning into a season-ending 18-team event – so this tie will no longer determine a relegation.