Groves was forced to abandon a world title shot against WBO super-middleweight champion Robert Stieglitz on May 5 because of nose damage.
It was the second time since December the 24-year-old had been forced to withdraw from a fight, with his rematch against Kenny Anderson falling through due to a back problem,
Eager to return to the ring for a first outing since hammering Paul Smith in two rounds last November, Groves insists he has emerged from a "dark place".
"It's been very frustrating, I was supposed to be fighting before Christmas, March and then May...it felt never ending," the British and Commonwealth champion said.
"When I was out in Cyprus I couldn't spar any more because my nose was busted.
"I was waiting on doctors and I was told to come to terms with the fact I might not be boxing for the world title.
"I tried desperately not to think about what it would be like to be a world champion, but you can't help yourself.
"I was in a dark place - sitting in a dark room in a villa in Cyprus wanting to cry. It all came crashing down really quickly
"We tried really hard to get a fight - any fight - as soon as possible and Sunday was the best date.
"I'm really excited and it will feel like a relief when I step into the ring. I'll be up for it because I have to be."
Little-known Sierra, also 24, has lost five of his 31 fights and should provide Groves with his second victory on American soil.
If Groves delivers the expected victory, he is being lined up to face Anderson at Wembley Arena on September 14 after climbing off the canvas to stop the Scot in the sixth in 2010.