Former Wimbledon champion Boris Becker has questioned Andy Murray’s mental toughness after he slipped to fourth in the ATP rankings.

Murray is expected to be overtaken by Novak Djokovic in the standing when they are published next week after the Serbian ace won the China Open title on Sunday.

And Becker – who won six grand slam titles in his career – believes Murray lacks the killer instinct possessed by rivals Roger Federer, Rafael Nadal and Juan Martin del Potro.

“The top places have been occupied by Federer and Nadal for some time and now along comes Del Potro,” said Becker.

“What they all have is that necessary killer instinct to make the most crucial points their best points.

“I think Andy has that in some tournaments, but for various reasons once he's in the quarter-final that killer instinct goes missing.

“It's all about the mental approach. Federer has it, Nadal has it and Del Potro has it. But Murray? I'm not sure yet.”