Top-seeded Nalbandian, who helped Argentina advance to the Davis Cup quarterfinals last weekend, seems to have sprained an abdominal muscle and will need three weeks for recovery.

Nalbandian’s compatriot, Gaston Gaudio is also sidelined with an injury in his right shoulder and will not be able to defend his title in the Argentine capital.

Meanwhile, American world number ten, Andre Agassi pulled out of the SAP Open in San Jose, as he is troubled with a recurring lower back strain.

The 35-year-old star “experienced severe pain in his lower back and down his legs” during a practice sesssion on Sunday, the tournament’s official website reported on Monday.

The second-seeded American, who missed last month’s Australian Open through injury, returned to play last week in Florida where he lost to Guillermo Garcia-Lopez in the quarterfinals.

Agassi, who won the SAP Open on five previous occasions, will be replaced by a lucky loser in the draw.