Ryan O'Shaughnessy and Aquabatique were the final two acts to be voted into Saturday's final by the public in this evening's last live semi-final.
However, one more act will be picked by the judges to join the pair and fellow finalists Ashleigh & Pudsey, Only Boys Aloud, Jonathan & Charlotte, Kai & Natalia, Loveable Rogues, Molly Rainford, Sam Kelly and Nu Sxool.
Speaking on Britain's Got More Talent, Cowell explained: "I just had a feeling that we'd be in a position where we had a certain act that we thought deserved a place, and I like breaking the rules.
"We're going to go upstairs after this and make a decision."
Cowell then stated that he already knew the act that he wants as the wildcard, a statement that was echoed by Amanda Holden and Alesha Dixon.
When asked who she thinks may win the entire competition, Holden said: "It will be between Pudsey and Jonathan & Charlotte."
However, Cowell disagreed, instead choosing tonight's victor O'Shaughnessy. He said: "I think this is the most open competition we've ever had.
"I think that the dog was the favourite, and Jonathan & Charlotte were amazing. But I've seen this happen before, where someone who is not on the radar wins. His songs and lyrics are really working with the audience. I think he's got a shot."
Britain's Got Talent continues on Saturday (May 12) for the final, airing at 7.30pm on ITV1.
Photo gallery - Britain's Got Talent's fifth semi-final:
Copyright: Rex Features Ken Mckay/Thames