World number seven Juan Martin del Potro has insisted that he is "happy" with his current ATP ranking.
The Argentine's progression since winning the 2010 US Open has been stilted, mainly by injury but also by inconsistency.
He is known for punishing power from the baseline, but admitted that other aspects of his game need fine tuning if he is to realise his full potential.
"I know that I can improve," Del Potro told reporters.
"But the top four are there for a reason, they are playing better than me, I know that I can beat them, but I need to improve my level.
"That's what I'm working on, like trying to come to the net more. Right now, I'm happy with my ranking, I'm not frustrated."
Del Potro beat Ernests Gulbis 7-6[5] 6-3 to advance to the quarter-finals of the Rotterdam Open, where he will face Jarko Nieminen.
