In a must-win game, England ran out 1-0 winners and progressed from Group C as runners-up after Landon Donovan's last-gasp winner for USA against Algeria took the Americans top.
- Slovenia 0-1 England: Jermain Defoe's strike sends England through to knockout stage
- Comment: Matthew Upson, are you ready for the game of your life?
When asked about it, the West Ham defender was delighted to have ensured England's safe passage to the knockout stages.
"It was just a reaction really, the ball got played into the box and it was flicked on to the inside of me and I just took the opportunity," Upson told BBC Sport.
"It had to be timed right and I was really pleased I was able to.
"There were lots of opportunities in the game and every minute we had to make decisions and there was always something to think about. It was just the reaction that was pleasing."
Upson agreed that the displays against USA and Algeria were below-par for England, but was pleased that there was an improvement in the performance as well as the result against the fellow Europeans.
"I think it was noticeable in the first couple of games we weren’t flowing the way we can do and I thought we showed that in patches today," he noted.
Having put on a solid display alongside John Terry to deny Slovenia with several fantastic blocks, Upson revealed that England had approached the game and treated it like a Premier League match.
"We played with a bit of freedom and the attitude we showed was spot on, we went about our job as a Saturday in the Premier League," he said.
"They [Slovenia] started hitting long balls and it started getting scrappy but we are more than used to dealing with that, and I think we did well to see it through."
England face an anxious wait to see who they will face in the last 16, with a potential clash against Germany possible.