It's only been a couple of months since Kei Nishikori moved into the No. 5 ranking, and he's not yet comfortable with the number.

Once success arrived it came quickly for the 25-year-old Japanese player who stands out as one of the game's rising stars and one of its most modest.

Nishikori spent the latter part of 2014 making history. He became the first Asian man to reach a Grand Slam final at the U.S. Open and finished the year ranked No. 5, the highest ranking ever achieved by a man from Asia.

At this year's Australian Open he has powered through the first four rounds to secure a spot in the quarterfinals and has tried to explain on a few occasions what it is about the number five that makes him uneasy.

"I'm really new to be No. 5, so I'm still not comfortable," Nishikori told the cheering crowd at Rod Laver Arena on Monday, where Japanese flags were waving during the windy afternoon match as he beat the supremely athletic No. 9-ranked David Ferrer 6-3, 6-3. 6-3.

"I think I feel more pressure than before," he said earlier in the week, describing "the different feeling" of being on the pedestal with the world's best players. "I think I need more time to get used to it."

Asked on Monday what number he might feel more comfortable with, Nishikori smiled. "Maybe like, 15 or 20."

There is a disconnect between the confident, aggressive Nishikori fans witness on the court and the self-deprecating person he becomes off the court.

When he plays, Nishikori is aggressive, quick and energetic, with leaping forehands and an ability to take the ball early, which adds to his power and gives his opponent less time to react. He is also extremely focused, confident and calm.

"That's what I'm really trying to do, (be) patient always but at the same time aggressive," said Nishikori, who broke into the top five after adding Michael Chang, the 1989 French Open champion, to his coaching team last year.

Nishikori is a star in Japan but has been based since age 14 in Florida, where he moved to train at the Nick Bollettieri's famed tennis academy.

Living in the U.S. makes it easier for Nishikori to escape the limelight, but he says Japan still feels like home.

"In Florida nobody talks to me," Nishikori said. "In Japan ... a lot of people recognize me. It's not easy to walk on the street."

"I feel more comfortable living in Japan. They have much better food," he smiled. But in Florida, "they have great facilities... good players I can train with. They have everything."

Maybe after he retires, Nishikori said, he will move back to Japan permanently. Not that retirement is on his mind just yet.

On Wednesday, he faces defending champion Stan Wawrinka in the quarterfinals.

It will be a rematch of their U.S. Open quarterfinal which Nishikori won in a gripping five sets before going on to beat No. 1-ranked Novak Djokovic in the semifinals. He has only reached the quarterfinal in Melbourne once before — in 2012 — and followed that with consecutive exits in the fourth round.

Wawrinka said he was bracing for another long, tough match against Nishikori.

"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," said the No. 4-ranked Wawrinka. "We'll see how I'm going to deal with that."