DOHA, Nov 1 (Reuters) - Serena Williams explained her decision to pull out of next week's Fed Cup final on Sunday, saying she was physically and mentally exhausted.

The 28-year-old, who has not played for the United States in the team competition since 2007, withdrew on Saturday, just days after being named in the squad to face Italy.

"I definitely really wanted to go, but I don't think I would do well physically," Williams, who beat sister Venus to win the WTA Championships on Sunday, told reporters.

Williams, who will end the year as world number one after winning five matches in six days in exhausting heat and humidity in Doha, finished the event with her left thigh heavily strapped.

"It's very difficult to play such a mental, mental week and then come back and play again right now i'm just struggling in every aspect of my body."

Venus, with whom Serena won the Olympic gold medal last year, is not in the team either it is a major blow to American hopes of winning the competition for the first time since 2000.

"It's a very difficult decision that I actually couldn't  make myself," she said.

"I had to get input from my mum and my dad. People that are close to me had to just basically tell me what I needed to do as opposed to me making the decision."

The American team for the Fed Cup clash consists of Melanie Oudin, Alexa Glatch, Serena's replacement Vania King and doubles specialist Liezel Huber.