After crushing Hungary 4-0 in the last 16, the Belgians felt they had one hand on the trophy but unfancied Wales had other ideas and sent them packing 3-1 in the quarter-final.
There were World Cup debuts for a host of England players including 19-year-old Trent Alexander-Arnold, the likes of Harry Kane, Jesse Lingard and Jordan Henderson all rested by Southgate, who justified not bringing Kane on by saying "If we played Harry Kane for 50 minutes tonight and he takes a whack, we would have been foolish", post-match.
"Of course, if we don't win, I understand people would say whatever about the Belgium match as well". "And whoever we play, in a one-off game, is capable of beating us".
"Many different reasons", Southgate said of England's knockout struggles.
"I think there is a belief; we've been undefeated for a long long time", he said of a run of 22 games unbeaten since losing to Spain in Martinez's first game in charge in September 2016.
"We created openings to get something from the game, but we have to keep improving".
"The victory is a effect of a very good performance". Both teams have already been eliminated from Russian Federation 2018.
"We need to look at ourselves, we are a stronger group than we were before the game". "We've got the chance to be the team that changes that".
The semi-final would then likely see England up against Croatia or Spain whilst Belgium, after beating Brazil, would potentially have to play Argentina or France.
And Southgate, who has faced criticism for his weakened team against Belgium, is unconcerned about the pressure he is under at Russian Federation 2018 following his team selection, with Group G winners Belgium earning a tie against Japan.
Former Manchester United midfielder Januzaj broke the deadlock with a superb curling effort in the 51st minute and the manner of the goal celebration should have silenced any fears that Martinez's side were not interested in victory.
The Daily Telegraph chose to "look on the bright side" having avoided pre-tournament favourites Brazil in the quarter-finals but added some fans may question Southgate's decision to make wholesale changes to the starting line-up.
"The most important thing for me is the players are in the best physical condition for the game".