Former Wimbledon champion Virginia Wade feels Andy Murray still needs to improve technically and mentally if he is to make his Grand Slam breakthrough.
Murray, who recently turned 25, will bid again to break his major duck when he competes in the French Open, which starts on Sunday.
Wade, who will be working as a pundit on the tournament for British Eurosport, said: "There are still some areas of doubt that come back to haunt Murray.
"Mentally and technically go hand in hand.
"As he says himself the second serve lets him down a lot - his first serve is fantastic.
"I know from experience that people talk about it - in interviews etc - and that knocks your confidence of it.
"Another problem is people want him to play more aggressively but when he gets closer in to the baseline he gets out of his comfort zone.
"There's conflict in his mind between doing that and playing the game he likes to play.
"So much of tennis is to do with the mental toughness. Djokovic is extremely tough and Nadal. You won't find two players that are more mentally tough."
Wade, whose best effort at Roland Garros was a run to the last 16, is also concerned about Murray's form. He has lost three of just eight claycourt matches in the run-up to the French Open and has not won an ATP title since January.
"This year he seems to have been a bit in and out," she added, before calling into question how much of an effect new coach Ivan Lendl has had since the pair linked up at the start of the season.
"Everyone was talking about him changing coaches and working with someone as good as Lendl but he's not got much experience as a coach, I think, and traditionally Murray has always been very much his own coach."