With only Monday's (AEST) 9.7km individual time trial left, Contador is virtually certain to claim overall victory in Santiago de Compostella, adding to his Vuelta triumphs in 2008 and 2012.
It will be his sixth Grand Tour success after wins in the Tour de France in 2007 and 2009 and the 2008 Giro d'Italia, and eighth if one is to include the two he was stripped of, in 2010 and 2011.
The Spaniard tracked Froome (Team Sky), the 2013 Tour de France winner, all the way up the final 12km climb before accelerating past the Briton in the final 500m to win by 16sec and increase his overall lead to 1min 37sec.
Contador, who broke his leg less than two months ago during the Tour de France, said the battle with Froome had inspired him.
"To fight for victory against a rider with the record and quality of Froome motivates me to compete with the best in the world," he said.
"It's true that this season has been a challenge for me. Everyone knows that the Vuelta wasn't exactly my aim but as luck would have it I came here.
"Through hard work and some luck I've stayed in the top two throughout the race so I couldn't do any more than that. I just hope to keep the lead tomorrow."
Echoing the sentiments, Froome said he'd given everything on the stage, and had been impressed by Contador's tenacity throughout the race. Like the Spaniard, his season could well have been over after succumbing to injury at the Tour de France, but after a slow start can be proud to leave the Vuelta as runner-up.
Froome's pressing and accelerations on the final 12km-long HC Puerto de Ancares climb shed the entire field with even the likes of Valverde, Rodriguez and Italian Aru unable to follow.
But no matter what he tried, Froome could not rid Contador, 31, from his rear wheel. The Tinkoff-Saxo captain patiently waited for Froome to fully expend himself before streaking clear to snatch the stage win, and seal the overall victory.
Alejandro Valverde came home third, 57sec down and retained that overall position at 2min 35sec, while Joaquim Rodriguez and Fabio Aru rounded out the top five on the stage as the five best riders in the race came home exactly in their overall positions.
"I never felt like there was a real threat on my podium finish today. I had good legs and everything was more or less under control through the whole climb," Valverde said.
"Froome's pace change was really strong and there was a point when I had to stop and keep going at my own pace. The problem is, you never know if it's the time to stop following them, and they (Froome and Contador) actually sat up just 10 seconds later.
"I kept my own pace and managed the gap until the end. When Purito (Rodriguez) was dropped I was able to accelerate and catch him, and when I did, I knew the podium was more or less done; on the final slopes I saw it was impossible to get back to the lead.
"For tomorrow's TT, I know more or less what you can see from the profile, though it's also known that there might be rain and it might become more risky. It will surely be beautiful for the fans to enjoy with us. We can make it to the overall podium again tomorrow; it would be my sixth time in the Vuelta and I'm really happy about it, as well as the great season we've done so far."
Earlier in the day, a four-man break consisting of Przemyslaw Niemiec (Lampre-Merida), Jérôme Coppel (Cofidis), Maxime Mederel (Europcar) and OPQS's Wout Poels opened up a lead of more than 10 minutes after the first 100km of racing, but Team Sky and Astana had little interest in allowing the move to stick.
The two teams worked together to bring the gap down and by the time the escapees began the penultimate climb, the 10km long first category Alto de Folgueiras de Aigas, their advantage had been shaved to just 3min on the peloton with 30km still to race.
By the time they reached the top the lead was down to just 43sec as the final summit, the Puerto de Ancares, loomed ahead.
Pole Przemyslaw Niemiec resisted the longest but once he was caught with 10km left, Sky's relentless pace at the front of the peloton had whittled down the leading group to less than 20 riders.
Irishman Philip Deignan dug in over a brutally steep section with an 18 per cent gradient that shredded the leaders down to nine riders before Spaniard Rodriguez sprung out of the group with nine kilometres left.
That attack saw the leaders split again with only the top five in the standings left fighting for the stage victory.
Rodriguez eked his lead out to 30sec but Froome then launched an attack that dropped first Aru and then Valverde, although Contador clung on grimly. That may have been subterfuge as the Spaniard then pounced, taking the stage win, and with it, sealing the overall title.
Stage 20: 185.7km, Santo Estevo de Ribas to Puerto de Ancares
1 Alberto Contador (ESP) Tinkoff-Saxo 5hr 11min 43sec
2 Christopher Froome (GBR) Sky 0:00:16
3 Alejandro Valverde (ESP) Movistar 0:00:57
4 Joaquim RodrÃguez (ESP) Katusha 0:01:18
5 Fabio Aru (ITA) Astana 0:01:21
6 Warren Barguil (FRA) Giant-Shimano 0:02:51
7 Giampaolo Caruso (ITA) Katusha 0:02:55
8 Samuel Sanchez (ESP) BMC 0:02:58
9 Daniel Navarro Garcia (ESP) Cofidis 0:03:15
10 Damiano Caruso (ITA) Cannondale 0:03:20
General Classification
1 Alberto Contador (ESP) Tinkoff-Saxo 81hr 12min 13sec
2 Christopher Froome (GBR) Sky 0:01:37
3 Alejandro Valverde (ESP) Movistar 0:02:35
4 Joaquim RodrÃguez (ESP) Katusha 0:03:57
5 Fabio Aru (ITA) Astana 0:04:46
6 Samuel Sanchez (ESP) BMC 0:10:07
7 Daniel Martin (IRL) Garmin-Sharp 0:10:24
8 Warren Barguil (FRA) Giant-Shimano 0:12:13
9 Daniel Navarro (ESP) Cofidis 0:13:09
10 Damiano Caruso (ITA) Cannondale 0:13:15
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