French 18th seed Gilles Simon's Australian Open hangs on an x-ray to determine whether he has broken his foot.
Simon is on crutches after suffering the injury in the semi-finals of the AAMI Classic at Melbourne's Kooyong.
He was trailing compatriot Richard Gasquet 7-6 (7-3) 1-0 when he appeared to roll his right ankle as he ran toward the net.
The 29-year-old was in obvious pain, throwing his racquet away and limping to his chair.
He couldn't continue and was assisted from the court with the joint heavily iced.
Tournament physiotherapist Rob Hanna said Simon would go for an x-ray with fears he's cracked his fifth metatarsal bone or suffered tendon damage.
"If he's cracked his metatarsal he will be in a moon boot and will be out for six weeks," Hanna said.
Simon was drawn to play German Daniel Brands in the opening round of the Australian Open.
Gasquet himself pulled out of Saturday's AAMI Classic final against Czech world No.7 Tomas Berdych, citing general soreness.
His place will be taken by Japanese world No.17 Kei Nishikori.
In other Kooyong matches, Australian teens Nick Kyrgios and Jordan Thompson finished with mixed results but still impressed against highly-rated opponents.
Playing his first match of the year Kyrgios, 18, faced Spanish world No.33 Fernando Verdasco and had a break in the third before succumbing 7-6 (7-2) 3-6 7-6 (7-4).
After almost toppling Berdych on day one at Kooyong, Sydneysider Thompson overcame Argentine world No.42 Juan Monaco 6-4 6-2.