The second-seeded Nadal won 6-1, 7-5 as the former top-ranked player continues his comeback from a long-term knee injury and builds toward the French Open.
Third-seeded Almagro also had a straight-sets win, defeating Horacio Zeballos 6-3, 6-4.
Nadal is making a comeback after being away from the tour for more than seven months with a left knee injury. He made the final of the VTR Open earlier this month in Chile before losing to Argentine Horacio Zeballos and a week later beat David Nalbandian to claim the Brazil Open title.
"So far I've played in three semifinals and that's fantastic news and it's even better because my three best games have been here," Nadal said.
"Now I feel like I have nothing to lose no matter what happens tomorrow. Either I will go on to play the final or I will go happy to play in another tournament knowing that the knee is responding better."
Nadal has good reason to be confident of beating Almagro, as he boasts an 8-0 record against his fellow Spaniard, yet the former No. 1 said his long absence from the tour must be taken into account.
"I don't feel as the favorite for tomorrow, it's been seven months without competing and I will be facing a great clay player," Nadal said. "I will do my best to play a great match to reach the final, it*s that clear."
On Tuesday, Nadal said he may pull out of next week's Indian Wells Masters in California to spare his fragile knees from the hard courts and save himself for the European clay season.
He didn't say whether he would also compete in the hardcourt Key Biscayne Masters, where he's reached the final three times, and which follows Indian Wells.