Lionel Messi looks "stressed" but will step up in Argentina's crunch World Cup clash against Nigeria, according to Pablo Zabaleta.

Argentina must defeat Nigeria, who they lost to 4-2 in Russia last November, and hope Croatia can do them a favour against Iceland to keep their hopes of advancing from Group D alive.

Messi had a penalty saved as Jorge Sampaoli's men were held by Iceland in their first outing, while the Barcelona star was a peripheral figure in the 3-0 humbling at the hands of Croatia.

Zabaleta thinks Messi is unfairly given a hard time by the Argentine public and backed him to come to the fore in St Petersburg on Tuesday.

"I think it is getting harder for Leo to play for Argentina. He turned 31 on Sunday so he is getting older and sometimes, mentally, you can feel tired as well," Zabaleta wrote in his BBC Sport column.

"Part of that is down to his history with Argentina. This team has lost the past three finals they have played - the 2014 World Cup and the Copa America in 2015 and 2016.

"In Argentina, that is not enough. For some people, and for parts of the media, you have to win - just reaching the final is not acceptable.

"So losing three finals in four years has been tough mentally and psychologically for everyone in that team, especially Leo.

"It is hard when people in your country treat you like a loser. They see him winning trophies every season with Barcelona - so they expect him to do exactly the same in the national team.

"Leo gets criticised when he does not play well, but when he is not around - like when he briefly retired from international football in 2016 - there are people calling for him to come back.

"As his team-mate, I can say without any doubts that we have always been a better team when he has been there.

"And those who say he does not perform for his country like he does for his club are wrong. When I played in the Argentina team with him, he was always the main player and the main man.

"I am sure he will be on Tuesday - because this could be his last chance."

Messi was pictured with his head in his hand prior to kick-off against Croatia and Zabaleta has been concerned by his demeanor in Russia.

"The happiest I saw Messi when I played alongside him for Argentina was in 2012 - he scored a hat-trick against Brazil in New York, and he played the whole game with freedom and a smile on his face," added Zabaleta.

"His body language was completely the opposite before the Croatia game on Thursday. To see Leo looking so stressed and unhappy at this World Cup was a real worry.

"He is my friend as well as my former team-mate and I felt really sorry for him. It is very unusual to see him like that, but it was a sign of what was going on in his head.

"At times like this, there is a huge pressure on him because people expect too much, and for him to do everything for the national team.

"I was not surprised he struggled, in what was a disastrous defeat for us. As a player myself, I know that if you go into a game and you don't really enjoy it then it is very difficult to play well."