Federer may feel he's had enough - Pat Cash

Former Wimbledon champion Pat Cash compares Roger Federer to Australian batsmen Ricky Ponting and Michael Clarke who he believes "hung in there" too long at the end of their careers.

37-year-old Federer suffered a shock exit at the US Open to world number 55 John Millman.

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