Andre Agassi's injury problems continued as he was forced out of the San Jose Open with a lower back strain.
Agassi has played just three matches so far this year
The American, seeded second, experienced severe pain in his lower back and down his legs during a practise sesssion on Sunday.
Agassi, 35, missed much of last season through injury as well as last month's Australian Open.
He returned last week in Florida, where he lost to Guillermo Garcia-Lopez in the quarter-finals.