Champion mare Rachel Alexandra remains in a serious condition as she recovers from surgery related to complications involving the birth of her filly.

A statement from Stonestreet Farm said the 2009 United States Horse of the Year had surgery on Wednesday to repair an area of bruising in her small colon which was injured during foaling.

"We are solely focused on making sure Rachel is as comfortable as possible," her owner Barbara Banke said.

"She is showing the same strength and courage now that she demonstrated on the track. Our hearts are broken by this turn of events and I can only ask that you send your thoughts and prayers to Rachel now."

Veterinary surgeons say it is too early to for full prognosis.

Rachel Alexandra's filly by Bernardini was born on Tuesday and is being cared for by a foster mare.

In January 2012, Rachel Alexandra gave birth to a colt by dual Horse of the Year Curlin.