Belgium manager Roberto Martinez has praised the "togetherness" his side showed to ​overcome a stubborn Panama side in the opening match of their World Cup campaign.

A dull first half was soon forgotten about when Napoli star Dries Mertens fired Belgium into the lead shortly after the break, but all the headlines went to Manchester United's Romelu Lukaku after the 25-year-old scored an impressive brace before the final whistle.

Belgium v Panama: Group G - 2018 FIFA World Cup Russia

Martinez noted that Mertens' incredible volley was key to Belgium's result on Monday, adding that the team showed a lot of 'maturity' to secure all three points in the opening fixture in Group G.

"I'm delighted. It was exactly what we expected - there are no easy games at the World Cup," Martinez claimed after the match, quoted by Sky Sports. "We started very well but then became frustrated as the first half wore on.

"But I was delighted in the way the team reacted and the commitment everyone showed to keep a clean sheet.

"In the second half we found our composure and the goal from Dries Mertens, a great goal, was so important for us.

"In a World Cup, you have to play for 90 minutes and we had to be prepared to work hard, show maturity and real togetherness."

Belgium will take on Tunisia - ​who suffered a late defeat against England on Monday - in the second match of the tournament before facing off against Gareth Southgate's Three Lions next week.