Tennis might be entering a "transition period" from the days of Rafael Nadal, Roger Federer, Novak Djokovic and Andy Murray, according to former British players Greg Rusedski and Annabelle Croft.

The quartet, commonly known as the Big Four, have dominated tennis for over a decade with a combined 51 Grand Slam titles and 285 career titles.

Until last year, it was Djokovic and Murray who were mainly dominating but then their poor form and injuries coincided with the career resurgences of Nadal and Federer, both of whom ended 2017 with a combined 13 titles as they shared the four Grand Slams between themselves.

Federer continued to dominate tennis in 2018 as he won the Australian Open for a second straight year before becoming the oldest world No. 1 in tennis en route to winning the Rotterdam Open.

But despite a remarkable start to the year that saw him win a career-best 17 matches in a row, Federer has lost his last two matches against Juan Martin del Potro in the final at Indian Wells and his opening round match against Thanasi Kokkinakis in the Miami Open last month.

With Del Potro winning his first-ever Masters 1000 title at Indian Wells and American John Isner going on to do the same in Miami last week, former men's British No. 1 Rusedski believes a transition could be taking place with more new faces winning the top titles.

"It feels like we are in a transition period with the men's game. There have been three new Masters Series winners [Jack Sock, Del Potro and Isner]," Rusedski said, as quoted on Sky Sports. "Those events had traditionally been dominated by the big four and possibly [Stan] Wawrinka."

"It is very interesting times. I think we are going to see the transition in men's tennis over the next year or two. Still with the old names doing well but with some new players knocking on the door," he said.

Croft, a former women's world No. 24, is in agreement, stating it is now no longer predictable as to who will win events these days, especially with the number of absences from the sport.

"There is a huge difference between what is happening in men's tennis now and when we had that massive domination [by the big four]," Croft explained. "We haven't seen quite a lot of the big names [in recent months] like [Stan] Wawrinka and Andy Murray."

"It is a fascinating time in men's tennis because no longer at the beginning of an event can you predict what will happen. At the start of the year it looked like Federer was going to dominate once again but that is not quite happening. Rafa has had the hip problems and then there is the massive number of young players coming through the game, who are trying to take advantage of this changing period in the game," she said.

With Nadal expected to return for the clay-court season that begins this month, it will be interesting to see if the Spaniard is able to keep dominating the surface as he looks to keep his newly-acquired world No. 1 ranking.