Spain host Ukraine in Madrid this weekend as they continue their challenge for a fifth title.
The 27-year-old captured his second US Open title at Flushing Meadows by beating world number one Novak Djokovic in four sets on Monday.
Nadal said that he has "good feelings" despite a tiring hard-court swing, which also included wins at the Rogers Cup and the Cincinnati Masters.
"I have just made a big effort but I'm going to do whatever I can to help the team," Nadal told reporters.
"If it is on Friday it would be on Friday, if it is on Saturday, on Saturday, if it is on Sunday, on Sunday. I'm going to try to help with everything within my capabilities.
"I've got good feelings. What do you want me to say after having winning three tournaments in a row, my feelings couldn't be better."
Spain last won the Davis Cup in 2011.