Gil now holds an overall lead of 34 seconds

The Saunier Duval rider made the break on the Albula, the third and final climb, 14km from the end of the mountainous 212km stage.

He crossed the line almost 40 seconds ahead of German duo Jorg Jaksche and Jan Ullrich.

Gil takes the leader's jersey from compatriot Angel Vicioso who finished two minutes off the pace.

"This is probably the best ever win of my career because I beat big-name riders like Jaksche and Ullrich," Gil said afterwards.

"I have to thank my team mate Jose Angel Marchante because he attacked first on the climb and that set me up for my chance.

"When I jumped away Ullrich and Jaksche were already tired and that helped me get away."

With two more mountain stages on Friday and Saturday and a final 30-km individual time trial on Sunday, Gil know his advantage may not be enough to give him overall victory.

He said: "The next two stages are tough and then the time trial will decide the race.

"I'm not bad against the clock but I think I need an advantage of a minute and a half to win. That means I'm going to have to attack again before Sunday."

Friday's 233-km seventh stage from St Moritz to Ascona is the longest of the nine-day Tour.

It includes the Julierpass and the Lukmanierpass but ends with a flat final 50km.

Stage result

1. Koldo Gil (Spa/Saunier Duval) 5:49:52"
2. Joerg Jaksche (Ger/Wuerth) +36"
3. Jan Ullrich (Ger/T-Mobile) +40"
4. Jose Angel Gomez Marchante (Spa/Saunier Duval) +1:39"
5. Simon Gerrans (Aus/AG2R) +1:47"
6. Fraenk Schleck (Lux/Team CSC) +2:09"
7. Janez Brajkovic (Slo/Discovery Channel) +2:07"
8. Giampaolo Caruso (Ita/Wuerth) s.t.
9. Angel Vicioso (Spa/Wuerth) s.t.
10. David Canada (Spa/Saunier Duval) +3:29"