The `Uptown Girl` singer was admitted to hospital in Long Island, New York, last week to be treated for a congenial hip ailment and decided to have both joints looked at in order to stop himself backing out of later surgery.
His ex-wife Christie Brinkley said: "He did them both at the same time. Billy said `If I do it one at a time I know I`ll never go in for the other.` "
Christie - who has 24-year-old daughter Alexa Ray Joel with the singer - said her former spouse is recovering well.
She added to RadarOnline.com: "He`s really doing well. He`s really coming through with flying colours. "He`s making it look easy!"
Billy, 61, recently admitted he was looking forward to performing again after having the surgery.
The singer - who has been using a cane to assist with walking in the lead-up to the operation - said: "This is the longest time I`ve had off in I don`t know how long - and I had fun. But I`m ready to start playing again."