Roger Federer. Pic/AFP

Roger Federer said Thursday retaining his world number one ranking is no more than a "mini-goal" as he prepares his bid to defend the ATP Miami Open title. The 36-year-old Swiss star needs to advance to the quarter-finals in Florida to hold on to his number one ranking, or face being supplanted by old rival Rafael Nadal in top spot. However Federer, who was beaten by Juan Martin Del Potro in Sunday's Indian Wells final, said he is not attaching too much importance to hanging on to his ranking. "The number one ranking is in my head a little bit, but no more than that," Federer said.