The world number five is in the Mexican city for his third clay-court event in four weeks following his return to action for the first time since a seven-month lay-off.
The Spaniard reached the final in Vina del Mar in the first leg of his Latin American tour before winning the title in Sao Paulo.
Yet the 26-year-old says his knee is still not fully recovered and he still feels intermittent pain.
“It’s better and I’ve been able to compete, but it’s still bothering me,” he said.
“It’s better on some days and worse on others, which isn’t ideal for someone like me, because my idea of competing is based on giving it all at all times.
“The pain continues, but I can’t specify what the level of pain is,” he added. “It’s clear I am not 100 percent. But after all that time [off] I’m happy to return to compete.”
Nadal, seeded second behind new Spanish number one David Ferrer, faces Argentine qualifier Diego Schwartzman in the first round in Mexico.