The Japanese tennis star rose to number five in the world following a good run of form, including a spot in the US Open final at the end of 2014.
He is keeping that momentum going at the Australian Open, having booked a quarter-final spot against defending champion Stanislas Wawrinka in the quarter-finals with a 6-3, 6-3, 6-3 victory over David Ferrer.
Nishikori said: "It's just a number. But still, I'm really new to number five I'm just not comfortable. I was Top 10 early last year. This is pretty new for me. I need some time to get more experience."
He added: "I start thinking [about] other things outside the court. I might feel pressure, other things. But I think I need some more time to get used to it."
Wawrinka is undecided about his strategy when he takes on Nishikori. Ahead of their upcoming match he said: "He's a great shot maker. He can make winners. He's always taking the ball really early. It's always tough to play against him. We'll see how I'm going to deal with that."