Simon battled back from a set down to beat Murray 4-6 7-5 6-3 on Monday, with the three-times Grand Slam champion suffering from cramp in the second set.

"Having that happen after a set and a half on an indoor court where it's not particularly hot is not really acceptable," Murray, who has struggled to regain top form since undergoing two hip surgeries, told reporters.

"It's nothing to do with my hip. I just think the reality is I need to work harder. Obviously, there's certain things I can and can't do nowadays, I need to be a bit more careful with some of the training that I do.

"But I can certainly do more than what I have done and push myself harder than what I have done recently. What I'm trying to do is extremely hard. I need to do exceptional things to still compete."

Murray has risen to No. 48 from 134th in the rankings this year and reached the finals in Sydney and Stuttgart in the first half of the season.

He said his dip in form in recent months could have been avoided by opting for a dedicated training stint instead of playing in Newport, Rhode Island immediately after Wimbledon.

"If I'm being completely honest, I don't think the last four or five months that that's necessarily been happening, since the issue sort of started in Newport, Washington time," Murray said.

"Maybe one of the mistakes that I made was like playing Newport. I could have done a training block there to get in the heat and prepare better for the summer ..."