Former world number one Andre Agassi became the second player to pull out of the season-ending Masters Cup in Shanghai on Monday.
The American withdrew from the $4.45 million (2.5 million pounds) extravaganza after losing his opening group match to Russia's Nikolay Davydenko 6-4, 6-2 citing an injury to his left ankle.
Earlier world number two and French Open champion Rafael Nadal pulled out with ligament damage in his left foot.
Agassi badly sprained his ankle playing racquetball four weeks ago and was never in contention against Davydenko.
French Open champion Nadal is due to address fans at Shanghai's Qi Zhong stadium later on day two of the $4.45 million event to explain his decision.
The Spaniard's withdrawal is a massive blow to organisers of the lucrative event who had already lost Andy Roddick, Lleyton Hewitt and Marat Safin from the line-up.
Australian Open champion Safin gave up his place in the eight-man field last month as he continued to struggle with a knee injury.
The Russian was joined on the sidelines by world number three Roddick who was suffering lower back spasms.
Twice-champion Hewitt pulled out due to personal reasons -- his wife, television actress Bec Cartwright, is expecting the couple's baby in Sydney within the next two weeks -- but he has also been recovering from a groin strain and toe surgery.
Nadal's place will be taken by Argentina's Mariano Puerta, this year's French Open runner-up.
The 19-year-old Nadal, who had been due to play Argentine Gaston Gaudio later on Monday, had sat out the final two weeks of the season because of knee tendinitis.