British number one Laura Robson is not mature enough and does not work hard enough to be a constant threat to the top players in women's tennis, according to outgoing coach Zeljko Krajan.
The Croatian joined the Robson camp following last year's Olympics and his introduction appeared to be doing the trick as the 19-year-old claimed notable wins over Kim Clijsters and Li Na on her way to the US Open fourth round.
However, Krajan, who helped Dinara Safina reach number one in the world in 2009, has been unable to push Robson on this year. The teenager has disappointed since reaching the Australian Open third round in January.
The pair parted company last week ahead of the start of the Madrid Masters, where Robson defeated Agnieszka Radwanska to record her first victory over a top-five player.
Krajan admitted he expected more from Robson in the mental stakes. He felt it was best for both parties to move on.
"She was not mature enough to do this kind of work enough times for me. She needed to be more serious and to commit herself more," the 34-year-old told tennis betting news reporters
. "She had to do this every day, not only in the big tournaments."
While Robson has continued to climb the rankings this season, the world number 41 has struggled to live up to the hype after an impressive 2012.
She suffered her sixth first-round defeat of the year at the Portugal Open at the end of April and was part of the Great Britain side that failed to reach the Fed Cup World Group.
Robson underlined her credentials in Madrid before losing to former world number one Ana Ivanovic. It will be up to her new coach to ensure she realises her potential.
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