Roger Federer and Rafael Nadal are on top of the tennis world at the moment, but Pete Sampras has sent the grand slam greats an ominous warning.
Federer and Nadal have had it fairly easy in 2017, with World No.1 Nadal winning the French and US Opens, while World No.2 Federer was victorious at the Australian Open and Wimbledon.
Nadal's victory at Flushing Meadows has been described as one his easiest grand slams, with fellow big guns Novak Djokovic, Andy Murray and Stan Wawrinka all missing through injury.
Nadal didn't face a single player in the top 20 as he cruised to his 16th career grand slam title, the first time that's happened at a major since Sampras won Wimbledon in 2000.
But according to Sampras, Nadal and Federer will have things much tougher in 2018.
The 14-time grand slam champion has backed the likes of Djokovic and Murray to return with a bang in the new year and challenge for more major titles.
Murray won just one title this year at the Dubai Open and struggled for form in the other events. He lost in the quarter-finals at Wimbledon after struggling with a hip injury and has since indicated that he will not play again in 2017.
Similarly, Djokovic has also had a difficult year after dominating the game in the recent past. The Serb managed to win two titles in 2017 but has been struggling for form and fitness like the Scot.
The former World No.1 has also called time on his current campaign owing to an elbow injury and will only return in 2018.
"I think if you take that much time off, they're going to be very eager and determined [to succeed]," Sampras told The Independent.
"I'm sure Novak and Andy are going to be right in the mix next year and compete for majors."
"Can [Murray] be the same force as he was before? Without a doubt. I don't see anything stopping Andy from a tennis-playing standpoint. He's one of the best in the world.
"It's just a matter of playing well at the right time. The only thing that can hold him back is injuries, because he does rely so much on his movement.
"Once you can't move and grind out the matches like he can, it's tough for him. It's tough for anybody," the American explained.
Sampras also weighed into the debate about whether Nadal can catch Federer's record haul of 19 grand slams.
"Can Rafa beat Roger's record? Basically it's up to Rafa and how much he wants to play and how much he loves the game," Sampras, who is now third in the all-time list added.
"If he said: 'Hey guys, I'm gonna play until I'm at least 35', I'd say he's got a pretty good shot at doing it.
"There's the age gap [between him and Nadal]. Five years is quite a bit in tennis. It just depends on how much longer Roger wants to go on, if he's enjoying it and how his body holds up.
"I just think it's great for the sport to see the [revival] of their rivalry. It's quite remarkable," the 14-time major winner said.