Top-seeded John Isner lost 6-4, 6-4 to fellow American Ryan Harrison in the second round of the Sydney International on Wednesday.
The exit of Isner, who was promoted to the No. 1 seed after Jo-Wilfried Tsonga and Richard Gasquet withdrew, followed two more injury withdrawals: No. 2 and 2011 Sydney champion Giles Simon (neck) and No. 6 Radek Stepanek (back).
Isner, who withdrew from the Hopman Cup mixed team competition in Perth last week with a right knee injury, lasted 71 minutes against No. 64-ranked Harrison.
The Australian Open starts Monday, leaving injured players little time to prepare for the season's first major.
In the women's draw, top-seeded Agnieszka Radwanska of Poland advanced to the semifinals with a 6-4, 7-5 win over Italy's Roberta Vinci.