Basque - Alberto Contador protected his lead of the Tour of Basque Country before the decisive time trial, while Ben Swift of England won Friday's fifth stage.
Swift edged a pack of riders at the finish line to claim the 160-kilometre leg featuring five categorised mountain climbs from Eibar to Markina-Xemein in just under four hours.
Alejandro Valverde, who is second overall, also finished second in the stage, a wheel-length behind Swift.
Contador finished in the leading group to make sure he didn't lose any of his 12-second lead.
The 54th edition of the six-day race in northern Spain concludes on Saturday with a 24-kilometre time trial to and from Beasain.
STAGE 5 RESULTS
1. Ben Swift (Britain / Team Sky) 3:56:57"
2. Alejandro Valverde (Spain / Movistar) ST
3. Michal Kwiatkowski (Poland / Omega Pharma - Quick-Step)
4. Tom-Jelte Slagter (Netherlands / Garmin)
5. Damiano Cunego (Italy / Lampre)
6. Cyril Gautier (France / Europcar)
7. Jonathan Hivert (France / Belkin)
8. Jean-Christophe Peraud (France / AG2R)
9. Alberto Contador (Spain / Tinkoff - Saxo)
10. Cadel Evans (Australia / BMC Racing)
11. Arnold Jeannesson (France / FDJ.fr)
12. Thibaut Pinot (France / FDJ.fr)
OVERALL STANDINGS
1. Alberto Contador (Spain / Tinkoff - Saxo) 20:30:31"
2. Alejandro Valverde (Spain / Movistar) +12"
3. Damiano Cunego (Italy / Lampre) +36" 4. Cadel Evans (Australia / BMC Racing)
5. Jean-Christophe Peraud (France / AG2R)
6. Yury Trofimov (Russia / Katusha)
7. Michal Kwiatkowski (Poland / Omega Pharma - Quick-Step) +41"
8. Mikel Landa (Spain / Astana) +54"
9. Wout Poels (Netherlands / Omega Pharma - Quick-Step) +55"
10. Samuel Sanchez (Spain / BMC Racing) +56"
11. Tom-Jelte Slagter (Netherlands / Garmin)
12. Thibaut Pinot (France / FDJ.fr)