Inexperienced Algeria were exposed by a much fitter Belgium side and ran out of gas in falling to a 2-1 World Cup defeat against the Group H favourites on Tuesday, coach Vahid Halilhodzic said.
The north Africans took the lead in the first half when playmaker Sofiane Feghouli fired home from the penalty spot but conceded two goals in the final 20 minutes.
Algeria were resilient opponents in the first half, restricting Belgium to few chances but as the game wore on they allowed the Europeans to take control, with Marouane Fellaini scoring with a superb header and Dries Mertens drilling home the winner 10 minutes from time.
"We let them play in the second half," Bosnian Halilhodzic told reporters.
"We should have run a little bit more, we should have dared a bit more. We didn't think Belgium would score this second goal. Physically, the Belgium players are much stronger."
Halilhodzic pinpointed a lack of physical conditioning as a reason for his side's late capitulation.
"My players were lacking in the second half, some of them were asking to be substituted," he said.
"The second half showed the difference in experience. They showed freshness, we committed small faults. But there was a lot of quality in our play.
"I have a number of players that are rather limited, they're young, they're not that experienced, they can't play 90 minutes. Each Algerian player needs to improve his physical condition by 30 to 50 percent. The Belgian players are in much better shape.
"But in spite of everything, we could have won. My team gave as much as they could in the present time."
Algeria's two other group games are against Russia and South Korea, who play each other later on Tuesday.