United Kingdom, July 9 -- Kelly Clarkson was "devastated" to postpone the opening of her highly anticipated Las Vegas residency,

The 43-year-old Grammy winner had been set to kick off her Studio Sessions residency at The Colosseum at Caesars Palace on 4 July, with 18 shows scheduled through to 15 November. But just hours before taking the stage on Independence Day, Kelly announced she would postpone the opening two performances.

A source has now told People the decision came after intense rehearsals left her with vocal strain that threatened long-term damage.

The insider said: "She was devastated and felt terrible for fans that had travelled to see her.

"She didn't want to cancel any shows. She's been very excited about the Las Vegas ...