Not everyone in men's tennis' so-called "Big Four" is surging. In fact, the "Big Four" are down to the "Big Three" for now.

Reuters is reporting that 25-year-old Japanese star Kei Nishikori has replaced Great Britain's Andy Murray at No. 4 in the world rankings. Rafael Nadal, who had dropped to fourth behind Murray, is back up to No. 3 after winning the Argentina Open over the weekend.

Andre Agassi called Andy Murray a "tortured perfectionist"

Nishikori moved up, based on his finals appearance in the Acapulco Open, where he lost to Spain's David Ferrer.

"What's important is where you are at the end of the year," said Nishikori, according to The Guardian. "I hope I can produce some big results."

Novak Djokovic seeks out Andy Murray to explain Australian Open third-set fatigue issue

He will be in action for Japan later this week in a Davis Cup World Group first-round clash against Canada in Vancouver.

Murray, meanwhile, is coming off what the Daily Mail reported as a "bizarre" loss to 18-year-old Croat Borna Coric in the quarterfinals at Dubai.

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Former No. 1 tennis player Andre Agassi says, however, that Murray likely will rejoin his spot among the "Big Four," using his own career as a comparison to that of the Scot's.

The Guardian reported that Agassi believes Murray is at the same stage of his career that Agassi was after winning his third Grand Slam at the Australian Open in 1995. Agassi went on to win eight singles Grand Slam titles.

"It appears to me that what he's gone through is not much different from what I went through," Agassi told The Guardian. "Which is: he was let on to this dirty little secret; that winning a slam doesn't change anything. If anything, it makes you realize that this better be about something else. You have to figure out a way to engage with the game differently. You have to figure out a way to make yourself better, despite being the best. There are a lot of working components. And it doesn't help to have Djokovic on the other side of the net.

"I think it's likely [he will win more] because he's one of two or three people that should win. It's likely he is going to win again despite himself. I leave a lot of room for his best years still to be ahead of him. It may not be as many years as he could have had, but I think we will see his best still."