Ian Carter reports (BBC Radio 5 Live)

The American, who won the tournament in 1990, suffered a back injury while practising earlier this week but declared himself fit to face Spaniard Alex Corretja in their White Group round robin match.

Agassi won the opening set 7-5, but lost the next 6-3 and called it a day after dropping his serve to trail 2-1 in the third.

"I hit a swinging volley at 2-2 in the first set and something got stuck. After that it was very painful and I couldn't move properly.

"It's a spasm in the back and it has tightened up. I had five injections yesterday as well as a lot of treatment but the pain didn't go away," said Agassi.

His decision increases the chances of British number two Greg Rusedski playing a part in the competition.

Rusedski was pipped to an automatic place in the eight-man event by Russia's Yevgeny Kafelnikov, but travelled to Germany in case any of the players were injured.

World number two Marcelo Rios is also struggling with a back problem, which affected him during his defeat by Tim Henman on Tuesday evening.

The Chilean left-hander complained about the injury following his 7-5 6-1 but hopes to be fit for his second match on Thursday.

Sampras reaches semis

World number one Pete Sampras guaranteed his place in the last four by outplaying French Open champion Carlos Moya 6-3 6-3 in their Red Group clash.

Both players had won their opening matches but Spaniard Moya, who has a poor 1998 record in indoor tournaments, was swept aside in just 62 minutes.

Sampras struck 13 aces as he improved his chances of finishing as number one for a record sixth successive year.