They're ranked in the top 10 on the WTA Tour, but were left off the team announced Wednesday for the Sept. 13-14 matches on outdoor clay at Club de Campo in Madrid.
Selected for the Russian team were No. 4 Svetlana Kuznetsova, No. 10 Vera Zvonareva, No. 56 Ekaterina Makarova and No. 71 Elena Vesnina.
Spain will be led by Anabel Medina Garrigues, Carla Suarez Navarro, Nuria Llagostera Vives and Virginia Ruano-Pascual.
Sharapova missed the U.S. Open and the Beijing Olympics because of a right shoulder injury. Sixth-ranked Dementieva won gold in singles at the Olympics, and No. 7 Safina has reached the finals in six of her last seven tournaments.
The fifth-ranked Sharapova has a torn rotator cuff tendon, but isn't expected to need surgery.
In the 3-2 semifinal win against the United States, the Russian team included Anna Chakvetadze, Kuznetsova, Zvonareva and Vesnina.
Russia has won three of the last four Fed Cup titles.
Medina Garrigues is Spain's highest-ranked player at No. 26.
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