Argentina's Juan Martin del Potro serves during a practice session in Buenos Aires, Argentina, Wednesday, Sept. 12, 2012. Slowed by an injury to his left wrist, del Potro is expected to play when Argentina opens its Davis Cup tennis semifinal against the Czech Republic on Friday. (AP Photo/Natacha Pisarenko)

Photo by: Natacha Pisarenko

Argentina's Juan Martin del Potro serves during a practice session in Buenos Aires, Argentina, Wednesday, Sept. 12, 2012. Slowed by an injury to his left wrist, del Potro is expected to play when Argentina opens its Davis Cup tennis semifinal against the Czech Republic on Friday. (AP Photo/Natacha Pisarenko)