Lionel Messi and Cristiano Ronaldo “probably have one last chance” to win the World Cup, according to Manchester United manager Jose Mourinho.
Portugal captain Ronaldo, 33, and Argentina skipper Messi, 30, have multiple honours for their clubs but neither have lifted the World Cup.
“They both arrive at this World Cup probably as the last one,” Mourinho said.
“It would be the pinnacle of their careers.”
Real Madrid forward Ronaldo and Barcelona forward Messi have nine Champions League final wins between them and a further 11 La Liga titles.
Ronaldo helped Portugal win Euro 2016 but Messi has yet to enjoy a major success with Argentina at senior level.
Both players are appearing at their fourth World Cup finals.
Speaking to Russian broadcaster RT, Mourinho added: “Everybody knows for the last decade Ronaldo and Messi are the two best players on the planet.
“They have tried to give the best of themselves to their countries.”
Asked if this was their last chance to win the World Cup, the Portuguese said: “Cristiano is a superman. He can [play at another World Cup] if he wants.
“But four more years at the highest level? To come back to the next World Cup I feel it will be hard.
“Of course Lionel is younger and he can perfectly be in the next World Cup in four years time.
“But I would also think that to play every week – two or three matches per week for Barcelona and Real Madrid in La Liga, Champions League, national team – I think to resist to all of that and four years later to come to the highest level is hard.
“They’re both going to try to finish their careers with the only thing they don’t have.”
Ronaldo is in action on Saturday when Portugal play Spain in the eagerly-awaited Group B pairing in Sochi.