Singer Sarah instantly won her place in tonight's live show when Simon hit the Golden Buzzer for her back at the auditions.
But she failed to top the public vote, with viewers instead putting pianist Tokio Myers and kid comedian Ned Woodman through to Saturday's final.
Simon was visibly stunned by the result (pictured), and afterwards he shared his thoughts on ITV2 spin-off Britain's Got More Talent.
"Fair play to the two of them," he said. "Ned was really funny, I think Tokio’s a real star, but that’s what happens on these shows.
"I meant what I said, I do think Sarah’s probably the best singer we’ve ever had on this show."
This series has seen a major change to BGT's format, with the judges no longer getting a say in the final line-up - other than a single 'wildcard' place.
Simon said it's "probably good" that the power is in the hands of the public.
However, he admitted that if it was up to him, he'd have sent home 9-year-old Ned, who has proven a hit by roasting the judges in his comedy act.
"I would have picked Tokio and Sarah," Simon said. "Ned, he’s really funny, but I think at this stage in their lives it’s more important that those two go through to the final."
One caller then dropped the killer question, asking the BGT exec if he regrets giving the "full decision to the public".
"No, no," he insisted. "Look, we have a wildcard. Even though I’m a bit pissed off, look, it is what it is, these things happen."
There are two more semi-finals to go before the judges decide which eliminated act will return as their wildcard.
Britain's Got Talent continues tomorrow at 7.30pm on ITV.