Agassi has pulled out of clay events to concentrate on Wimbledon

The 35-year-old pulled out of the Nasdaq-100 Open in Miami on Friday with a recurrence of a chronic back problem.

"It seems pretty dark at the moment," he said. "Is it possible that I can't anymore? It's definitely possible."

The six-time champion, who turns 36 next month, withdrew just hours before he was due on court to face Christophe Rochus on Friday.

The back problem has plagued the 10th seed all season and was part of the reason behind him opting out of all European clay-court events this year.

I am an optimist at heart Andre Agassi

"It's been the same story for a while. It's frustrating and exhausting and I'm way less than 100%," said the American.

"There are a couple of good days followed by a series of difficult ones. There are too many little setbacks. I don't just want to play, I want to play to win.

Agassi has pulled out of this year's clay events to concentrate on Wimbledon.

Miami was to have been only his fourth event of the season after missing the Australian Open with an ankle injury.

However, he has insisted he will return to the tour once he is fit again.

"I am an optimist at heart, and I will believe in more moments that I can still have out there.

"I mean, when my body is right, there's still a considerable amount left in me, I believe that.