ATLANTA : Cristiano Ronaldo and Luka Modric will make history when two outfield players aged over 40 face off for the first time in a World Cup as Portugal take on Croatia in Thursday's last-32 clash.

But both men are facing questions over whether their status as national heroes is now more of a burden than a boost to their countries' hopes of progressing far in the competition.

Prior to this tournament, only Cameroon's Roger Milla had played in a World Cup as an outfield player after turning 40.

Ronaldo and Modric, who won four Champions Leagues in six seasons together at Real Madrid, have long defied Father Time but have shown their age in what will almost certainly be their final flourish on the global stage.

At 41, Ronaldo defiantly screamed "I'm back" after netting twice against Uzbekistan as he became the first player to ever score at six World Cups.

Yet either side of a 5-0 romp over a team ranked 60th in the world, Portugal failed to beat the Democratic Republic of Congo and Colombia and fell into a tougher section of the draw as a result.

"It's already 23 years I've been a professional and whenever things don't go well it's 'Cristiano, he's finished, he's old'," Ronaldo said earlier in the tournament.

The forward played every minute of the group stages and outgoing Portugal boss Roberto Martinez shows no sign of taking the bold call to sit the five-time Ballon d'Or winner on the bench.

"There is not an issue physically or mentally for Cristiano in today's game to play the 90 minutes," Martinez said after Ronaldo managed just two touches in the Colombia box.

- Protecting Ronaldo's ego -

After a slow start to the World Cup four years ago, Ronaldo was eventually dropped by Fernando Santos in favour of Goncalo Ramos.

The AC Milan striker immediately scored a hat-trick in a 6-1 demolition of Switzerland, but a 1-0 defeat to Morocco in the quarter-finals meant Ronaldo's time out of the side was short-lived and Santos was removed as coach.

The impression that Martinez is wasting a richly talented generation just to protect Ronaldo's ego grows with every pedestrian performance.

Portugal's depth of midfield talent meant Paris Saint-Germain's Joao Neves and Bernardo Silva, now of Real Madrid, started on the bench against Colombia.

Yet even with one of the World Cup's strongest midfields behind him, Ronaldo's strikes against Uzbekistan are his only non-penalty goals in his past 14 games at major tournaments.

Modric, 40, similarly showed his age in Croatia's tournament opener, which ended in a 4-2 defeat to England in Dallas.

Caught on his heels, Modric conceded a penalty when chopping down Noni Madueke for England's opening goal and was sacrificed by Zlatko Dalic before the hour mark.

Croatia bounced back to celebrate Modric's 200th cap, edging past Panama 1-0.

In doing so, Modric became the fourth man to reach a double century of international appearances, joining Ronaldo.

The AC Milan midfielder then provided the assist for Nikola Vlasic's winner as Croatia beat Ghana 2-1.

Finalists in 2018, Croatia also reached the semi-finals four years ago.

Modric was the driving force behind those incredible runs for a country of less than four million people.

A likely last-16 meeting with Spain and one final shot at glory awaits the winners in Toronto.

But for one of football's all-time greats, the World Cup curtain will come down in Canada.