Germany's Jens Voigt produced a superb solo ride to win the first sector of the fifth and final stage of the Tour of the Basque Country on Friday.
Osa goes into the final time trial with a slender lead
The CSC rider attacked just 10 minutes into the leg and came in more than a minute ahead of Spain's David Blanco.
Spain's Aitor Osa retained the race lead, just one second ahead of Italy's Danilo Di Luca.
But the Basque rider will face a tough task to emerge victorious after the afternoon's final time-trial.
Stage five, first sector:
1. Jens Voigt (Ger, Team CSC) two hours 23 mins 24 secs
2. David Blanco (Sp, Comunidad Valenciana) at 1:15
3. Danilo Di Luca (It, Liquigas) at 1:34
4. Aitor Osa (Sp, Illes Balears) at 1:35
5. Miguel Angel Martin Perdiguero (Sp, Phonak) at 1:38
6. Damiano Cunego (It, Lampre-Caffita) same time
7. Cadel Evans (Aus, Davitamon-Lotto)
8. Bobby Julich (US, Team CSC)
9. Michael Rogers (Aus, Quick-Step)
10. Oscar Pereiro (Sp, Phonak)
Latest standings (Fri):
1. Aitor Osa (Sp, Illes Balears) 18:00:00
2. Danilo Di Luca (It, Liquigas) at one second
3. Davide Rebellin (It, Gerolsteiner) at 5 secs
4. Damiano Cunego (It, Lampre - Caffita) at 8 secs
5. Joaquin Rodriguez (Sp, Saunier Duval) at 10 secs
6. Michael Boogerd (Neth, Rabobank) at 15 secs