Brasilia: Cristiano Ronaldo's Portugal face a sobering early World Cup exit even if their world footballer of the year produces pyrotechnics in their final group game against Ghana.

The Real Madrid player came to the World Cup alongside Argentina's Lionel Messi and Brazil's Neymar as one of a triumvirate of true global superstars.

While the Barcelona pair have produced moments of sublime skill and goals, the attention surrounding Ronaldo has mainly focused on his troublesome left knee.

Portugal are rock bottom of Group G with a solitary point from their matches against Germany and the United States, both of whom have four points.

Cristiano Ronaldo during Portugal's training session on Monday. Pic/AFP
Cristiano Ronaldo during Portugal's training session on Monday. Pic/AFP 

They were pummelled 4-0 by Germany and they still have any chance of progressing at all is down to Ronaldo, who delivered a sublime cross for their equaliser in a 2-2 draw against the United States.

Despite that goal, if United States and Germany draw, Portugal and Ghana, who also have one point, are doomed.

"I never thought we would be world champions. We have to be humble and recognise our limitations. At the moment there are better teams and better players than our own," said the Portugal captain.

Portugal's task is made even more difficult by their goal difference of minus four, meaning they must score a hatful of goals to be in the mix. Ghana's goal difference is minus one.

And they are unlikely to have it all their own way when they take on Ghana in Brasilia after the Black Stars gave Germany a fright before Miroslav Klose equalised to rescue a 2-2 draw.

Despite that setback Ghana, who reached the quarter-finals in South Africa in 2010, still harbour hopes of progressing to the round of 16.