Dutchman Thomas Dumoulin gained a three-second lead in the Vuelta a Espana after a superb time-trial.
Dumoulin, 24, completed the 38.7km course in Burgos in 46 minutes one second, more than a minute faster than Maciej Bodnar and Alejandro Valverde, who finished second and third.
Italian Fabio Aru, second overall, lost only 1min 53secs to Dumoulin and is expected to do well in the mountain climbs over the final four days.
"This race is not finished," said Aru.
Aru, 25, who won the mountainous Giro d'Italia stage 19 summit finish this year, added: "I will be saving my best until the end."
Dumoulin, who is trying to become the first Dutch rider to win a Grand Tour since Joop Zoetemelk was victorious in the 1980 Tour de France, was surprised by his wide margin of victory.
"The course suited me but to win by over a minute is amazing," he said.
"It - the overall lead - is only three seconds so I am a little bit worried. A small gap in a bunch finish is three seconds.
"It is going to be very stressful but, with two stage wins, the Vuelta has already been a success for me."
The overnight leader, Joaquim Rodriguez, could only finish joint 30th in the time-trial and is now 1:15 back in third overall.
The 21-stage Grand Tour finishes in Madrid on Sunday, 13 September.
Stage 17 result:
1. Tom Dumoulin (Ned/Giant) 46mins 1sec
2. Maciej Bodnar (Pol/Tinkoff-Saxo) +1min 4secs
3. Alejandro Valverde (Spa/Movistar) +1min 8secs
4. Vasil Kiryienka (Blr/Team Sky) +1min 31secs
5. Jerome Coppel (Fra/IAM Cycling) +1min 32secs
6. Nairo Quintana (Col/ Movistar) +1min 33secs
7. Romain Sicard (Fra/Europcar) +1min 36secs
8. Nelson Oliveira (Por/Lampre) +1min 38secs
9. Steve Cummings (Gbr/Team MTN) +1min 40secs
10. Fabio Aru (Ita/Astana) +1min 53secs
General classification after stage 17:
1. Tom Dumoulin (Ned/Giant) 68hrs 40mins 36secs
2. Fabio Aru (Ita/Astana) +3sec
3. Joaquim Rodriguez (Spa/Katusha) 1min 15secs
4. Rafal Majka (Pol/Tinkoff - Saxo) +2min 22secs
5. Nairo Quintana (Col/Movistar) +2mins 53secs
6. Alejandro Valverde (Spa/Movistar) +3mins 15secs
7. Esteban Chaves (Col/Orica) +3min 30secs
8. Daniel Moreno (Spa/Katusha) +3mins 46secs
9. Mikel Nieve (Spa/Team Sky) +4mins 10secs
10. Louis Meintjes (SA/ Team MTN) +6mins 51secs