The Big Four era in tennis may soon be over. 

Speculation about who would break up the dominant reign of Roger Federer, Rafael Nadal, Novak Djokovic and Andy Murray has gone on for months. However, instead of who, the better question might be what?

It's unlikely that a single player will emerge to signal the end of this era. More likely, the rise of a few players will coincide with the demise of the Big Four. 

The 2014 U.S. Open final featuring Marin Cilic and Kei Nishikori marked the first Grand Slam finale since 2005 without a member of the Big Four. 

Stan Wawrinka's win over Nadal at the Australian Open offered a hint at things to come. The Wimbledon semifinals appearances of Milos Raonic and Grigor Dimitrov served notice that the rising stars were ready to establish themselves among the elite.

Meanwhile, Murray is no longer in the ATP Top 10. He may even fail to qualify for the year-end World Tour Finals. Nadal is out with an injury. Federer, while playing inspired tennis, is showing his age. Djokovic, newly married, is reportedly fatigued. 

Everyone knew the time would come. However, it's arrived more quickly and quietly than we expected. We kept our ears prepared for a gong-like blast ushering in a new day in men's tennis. What we got was a softer, subtle chime announcing that change is on the way. 

Believe it. The Big Four era is on its way out. The following are the biggest reasons why. 

Roger Federer's Decline

Federer has looked better this year than he did in 2013. However, it's clear he's on the decline. No matter how many racket, string or coach changes Federer makes, Father Time is undefeated. 

Some people point to his contemporary on the women's side, Serena Williams, as proof that Federer can still win Slams. After all, the two were born just a month apart.

They ignore the difference in miles on their bodies. Federer has played 1,202 singles matches. Williams has played in just 795. Even if you add her doubles matches (202) that's still 200 fewer matches than Federer. That's nearly three years of additional wear and tear. 

Ranked No. 2, Federer still competes well enough to reach Slam finals. It's just that getting to them is no longer automatic. 

Rafael Nadal's Injuries

Nadal withdrew from the U.S. Open and the summer hard-court season with a wrist injury. A knee injury kept him away from the game for seven months in 2012. According to the Kazakhstan Tennis Federation (via International Business Times), he's scheduled to play in an exhibition in Kazakhstan later this month. 

The dude is banged up. The same high-octane brand of tennis that won him 14 Slams may be beating his body into early retirement. How much longer can he play before breaking down completely? 

Veterans Breaking Through

Cilic, in an interview after winning the U.S. Open, said seeing Wawrinka win the Australian Open gave other players hope that they could break the Big Four's run on Slam titles.

Cilic became the second non-Big Four player this year to win a Grand Slam. Cilic, who turns 26 later this month, is among a handful of veteran players who are too old to be considered up-and-coming but young enough to have played most of their careers in the shadows of the Big Four. 

They too see the young guns closing in. Guys like Ernests Gulbis and Gael Monfils can see a window of opportunity, and they're anxious to pounce. 

Djokovic's Fatherhood Fatigue

Djokovic withdrew from Davis Cup, citing fatigue, according to the Associated Press (via The Guardian). The expectant father is also a newlywed, both major lifestyle adjustments. 

Historically, few singles players have managed to continue winning Slams after becoming a parent. Just two of Federer's 17 Slams came after his first set of twins was born. 

Ivan Ljubicic told ESPN in 2009 that fatherhood impacted his life and game. "It's just that your life changes completely. Priorities are different. You're not the one in the family who's the most important..." 

In the same article, Boris Becker added, "It would be more difficult to focus on one thing alone because your main focus is for your child and your wife, and tennis becomes secondary. That's a fact."

Djokovic will be a young father. However, the lack of life experience could make adjusting to fatherhood even more difficult. 

Rising Stars Soaring

Nishikori, Raonic and Dimitrov are all Top 10 players 25 or younger. Once rising stars, these guys are now soaring and challenging the Big Four.

Behind them are some even younger stars, such as Dominic Thiem and Nick Kyrgios. These players have shown they have the talent and are gaining the mental tools to compete for Slam titles. 

Nishikori has a 2-1 head-to-head record against Djokovic. Raonic has a 3-1 record against Murray. When an up-and-comer has a winning record against a member of the Big Four, future wins are no longer upsets. 

Andy Murray's Struggles

The biggest threat to the Big Four is Murray, the weakest link. He's no longer ranked in the Top 10 and could miss the year-end championships.

Without him it's the Big Three. 

Murray has played some bad tennis in Grand Slams. When he returned from back surgery, a slow start was expected. His poor performances at Slams were not. 

He has just one Slam more than Wawrinka, Cilic and Juan Martin del Potro. His struggles have opened the door for other players.