The Britain's Got Talent judge tweeted a note of congratulations to his rival for BBC One's ratings success over the ITV1 talent show last night (April 7).
"A slightly irritated congrats to Danny and the BBC," he wrote.
Referring to Britain's Got Talent's Ashleigh Butler and her dog Pudsey, he joked: "But you didn't have a dancing dog!"
However, Cowell also put out a message of support for his own show, adding: "I thought our team made a fantastic show last night and congrats to them as well. Loving the show this year."
The Voice UK attracted four million more viewers than Britain's Got Talent during their 20-minute crossover between 8pm to 8.20pm.
Simon Cowell was recently rumoured to have developed a crush on Britain's Got Talent make-up artist Francesca Neill.
> The Voice UK vs Britain's Got Talent: Our verdict