Alejandro Valverde and Joaquin Rodriguez, Contador's closest pursuers in the general classification, finished ahead of their fellow Spaniard but not with enough advantage to close the gap to the 2008 champion.
Contador crossed the finish 4 minutes, 15 seconds after Menchov to take his leading overall time to 82:14:52, which was 1:16 quicker than Valverde and 1:37 ahead of Rodriguez before Sunday's last stage, a largely processional 115-kilometre (71-mile) ride into Madrid.
Contador is on the verge of a fifth victory in cycling's major events to go with a pair of Tour de France triumphs and the 2008 Giro d'Italia, which he won before his first Vuelta victory later that same year.
"Yes, now the Vuelta is practically won," the Madrid native said. "It has been very difficult and we've had to hold our head high and our legs strong."
On the bending climb to the weather station atop the Bola del Mundo, Contador was just in front of Valverde and Rodriguez as the peloton eased from smooth pavement on to an uneven surface as throngs of fans choked the road.
Rodriguez made a dash with less than 2 kilometres left as the riders dug in and the peloton slowed to a crawl at times while navigating the 23 per cent angled climb to the 2,247-meter (7,372-feet) finish.
With Rodriguez ahead of Contador, Valverde also found reserves to pull ahead and finish behind Rodriguez but the difference was minimal and the pair failed to make a significant dent in Contador's advantage. Valverde managed to gain just 19 seconds while Rodriguez chipped off 48 seconds as Contador stuck to his own pace to the finish.
"Again the team was great on the Bola del Mundo," Contador said. "I knew Joaquin was going to attack but there wasn't enough terrain left for him to make up time. It's a special win."
Two-time Vuelta winner Menchov outlasted Richie Porte of Australia to win the penultimate leg by 17 seconds in 4:48:48. It was the Russian's fourth career stage win at the Spanish classic.
Kevin de Weert of Belgium was 42 seconds behind Menchov in third ahead of Sweden's Fredrik Kessiakoff and Romain Sicard of France, who was 1:39 behind.