The mountainous 16th stage was won by Italian Dario Cataldo of Team Omega Pharma - QuickStep.

Contador tried repeatedly to break away from his fellow Spaniard during the grueling final stretch after four special category climbs. Rodriguez pulled ahead as the riders neared the finish at the summit.

Rodriguez extended his lead by 6 seconds over Contador in the 21-stage race. He now leads the 2008 champion by 28 seconds as he nears what would be his first Vuelta victory.

Cataldo clocked 5 hours, 18 minutes, 28 seconds over a 114-mile stage that finished at the winter resort of Valgrande-Pajares, 6,100 feet above sea level.

Vacansoleil's Thomas de Gendt of Belgium was next, 7 seconds back. Rodriguez was 2:39 behind in third, followed by Contador.

"I've made a great step (to winning)," commented Rodriguez, whose overall time is now 63:38:24. "There's not much Vuelta left ... Luckily, I have these extra seconds under my belt."

Rodriguez sat on Contador's back wheel during the final 3.1 miles as the road narrowed with spectators lining both sides. The last 11 miles of the stage were all uphill.

"You exert so much on these ascents. You try not to empty yourself out too soon," Rodriguez explained. "Alberto controlled it pretty well though."

Contador, a two-time Tour de France champion, paid tribute to Rodriguez after his failed breakaway and knowing there are few opportunities left before Sunday's finish.

"We arrived together, we both tried. Independent of the result, I can't take anything away from Joaquin who has been so strong," said Contador, who missed this year's Tour because of a doping ban. "I'm happy with the work we've done. My legs were better today. I would have preferred a bad weather day, it could have helped me."

There has been little rain or wind during the 67th edition of the Spanish classic. Contador has one last chance to seize the leader's red jersey, on Saturday in the penultimate stage that ends with another testing mountain finish at Bola del Mundo.

"We've luckily had great weather so far. Let's hope it stays," added Rodriguez, the Giro d'Italia runner-up.

Alejandro Valverde (Movistar Team) couldn't keep up with his fellow Spaniards on the final climb, finishing 2:58 back in fifth place to slip 2:04 behind Rodriguez in the standings.

Tour de France runner-up Christopher Froome of Team Sky Procycling dropped out of contention after finishing 5:11 behind Cataldo. The rider from Great Britain is 4:52 back from Rodriguez.

Tuesday is the second and final rest day of this year's Vuelta a Espana - before Wednesday's 187.3 kilometer 17th stage from Santander to a mountain finish at Fuente De.

The 21-stage Vuelta concludes on September 9 in the Spanish capital Madrid.

Watch video highlights from all stages of the 2012 Vuelta a Espana in our videos section (U.S. only - due to geo broadcast rights restrictions).