Andy Roddick is the latest star to pull out of the Masters Cup in Shanghai.
Roddick was hurt during last weekend's Paris Masters
The world number three has been sidelined by an ongoing back problem, his place in the elite eight-man field going to Argentina's David Nalbandian.
"Though I have continued to practice, train and play, the lower back spasms continue," said Roddick.
Lleyton Hewitt, who is awaiting the birth of his first child, and Marat Safin, who has a knee injury, have already withdrawn from the event.
Roddick sustained the injury at the Paris Masters last Friday, during his quarter-final match against David Ferrer.
"After being evaluated by medical staff upon my return to the United States, it is now clear that I am not able to resume tennis activities for 10-14 days," he added.
Meanwhile, the draw has been made for the event which starts on Sunday.
World number one Federer lines up against Guillermo Coria, Ivan Ljubicic and Nalbandian in the red group.
French Open champion Rafael Nadal takes on Agassi, Nikolay Davydenko and Gaston Gaudio in the gold group.
Federer has been suffering with a torn ankle ligament and has not played since beating Andy Murray in the Thailand Open final on 2 October.
Nadal has been struggling with knee problems and Agassi has been inactive because a back problem since losing the US Open final to Federer.