24 Hour Fitness CEO Carl Liebert called Ted Forstmann a friend and mentor as he mourned the loss of the founding partner of private equity firm Forstmann Little and Co., 24 Hour’s parent company.

Forstmann died Sunday morning at the age of 71, seven months after he underwent surgery to remove a malignant brain tumor. A spokesperson for Forstmann Little and Co., told Bloomberg News that Forstmann died at his home in New York City surrounded by family.

Forstmann Little and Co. acquired 24 Hour in 2005 for $1.6 billion. The following year, 24 Hour hired Liebert as CEO.

“On behalf of 24 Hour Fitness’ nearly 22,000 team members, I would like to convey deepest sympathies to the family of Ted Forstmann, a visionary financial pioneer, astute businessman and generous philanthropist,” Liebert said today in a statement released by the San Ramon, CA-based company. “I appreciate the confidence he demonstrated in our business as an investor and in me by appointing me to oversee it on a daily basis. He’s been a brilliant mentor to me. I am grateful to have had the opportunity to work closely with Ted and to have known him both as a leader and as a friend. His dedication to improving the plight of those in need was inspirational and a truly enduring legacy. He will be missed daily, but in our thoughts, our hearts and our minds forever.”

24 Hour is No. 1 on Club Industry’s Top 100 Clubs list with an estimated $1.352 billion in 2010 revenue.

Forstmann also was the chairman and CEO of IMG Worldwide, a sports, fashion and media business he acquired in 2004.

“While known for his entrepreneurial spirit and business acumen, Mr. Forstmann was also a philanthropist in every sense of the word,” IMG said in a statement. “He was most passionate about children’s causes but rarely missed an opportunity to dedicate his time or make a financial contribution to other charitable organizations or goodwill groups.”