The Three Lions have made an unexpected charge into the last four of the tournament, with their 2-0 win over Sweden on Saturday booking their first semi-final appearance since 1990.
Croatia have also impressed throughout the tournament, though, and Alli is under no illusions that the 1998 semi-finalists will pose a difficult test.
"Obviously last night was a great night. We have to relax now, recover properly and prepare for the next game," Alli told Sky Sports News.
"As a team it was a solid performance by us. It was a difficult game, they've shown in their other results that they're a hard team to play against so to win the game comfortably with a solid performance - and obviously some great saves by Jordan [Pickford] - shows how far we're coming and how much we're improving as a team.
"It's going to be difficult - the semi-final at a World Cup, it's not going to be an easy game - but it's a game we feel we're ready for.
"We're confident but we know it's going to be very difficult. But the way the tournament's been going so far, whoever we come up against we feel confident we're going to win and we have to be like that if we're going to get into the final."
Should England beat Croatia then they would face either France or Belgium in the final.