Vinokourov took the overall lead after Wednesday's 17th stage

The Kazakh rider completed the 27.5km sprint from Rivas Futura to Rivas Vaciamadrid in 33 minutes, 39 seconds with Samuel Sanchez six seconds behind.

The win, on Vinokourov's 33rd birthday, extends his overall lead to more than a minute ahead of Sunday's final stage.

Briton David Millar had fancied his chances after winning the time trial on stage 14 but had to settle for 12th.

Vinokourov, who is aiming for the first major Tour of his career, is now one minute 12 seconds ahead of Alejandro Valverde going into the final day.

"I started cautiously but when I saw I could win the stage I stepped on the accelerator," said Vinokourov said.

"A victory like this on my 33rd birthday is a good present to myself.

"I had a rough first week here in the Tour of Spain, but after I won two stages in Lugo and La Cobertoria, things changed radically."

Stage 20 results:
1. Alexandre Vinokourov (Kazakhstan / Astana) 33:39"
2. Samuel Sanchez (Spain / Euskaltel) +6"
3. Alejandro Valverde (Spain / Caisse d'Epargne) +19"
4. Laszlo Bodrogi (Hungary / Credit Agricole) +26"
5. Vladimir Karpets (Russia / Caisse d'Epargne) +30"
6. Stuart O'Grady (Australia / Team CSC) +32"
7. Carlos Sastre (Spain / Team CSC) +44"
8. Vladimir Gusev (Russia / Discovery Channel) +49"
9. Thorsten Hiekmann (Germany / Gerolsteiner) +55"
10. Sebastien Rosseler (Belgium / Quick-Step) +1:00"

Overall standings:
1. Alexandre Vinokourov (Kazakhstan / Astana) 77:42:17"
2. Alejandro Valverde (Spain / Caisse d'Epargne) +1:12"
3. Andrey Kashechkin (Kazakhstan / Astana) +3:12"
4. Carlos Sastre (Spain / Team CSC) +3:35"
5. Jose Angel Gomez Marchante (Spain / Saunier Duval) +6:51"
6. Tom Danielson (U.S. / Discovery Channel) +8:09"
7. Samuel Sanchez (Spain / Euskaltel) +8:29"
8. Vladimir Karpets (Russia / Caisse d'Epargne) +10:36"
9. Manuel Beltran (Spain / Discovery Channel)
+10:47"
10. Luis Perez Rodriguez (Spain / Cofidis) +11:32"