"I just had some issues and went preventive to make sure I didn't make anything worse," said Williams. "I'm literally taking it one day, one match at a time, and not putting any pressure on myself."
Nadal, meanwhile, certainly fed his confidence with a 6-1, 6-0 victory over Juan Monaco of Argentina, just what the top-seeded Spaniard needed after coming to the Spanish capital on his worst clay run in a decade.
Nadal, the greatest ever player on the red surface, failed to advance past the quarterfinals of his last two clay tournaments. But after Monaco broke to get to within 2-1, Nadal did not relent as he didn't drop another game at Manolo Santana center court.
"Each victory helps to raise your confidence," Nadal said. "I started a little nervously in my movement. But I got better, and now I just have to prepare for tomorrow."
Nadal's hopes of winning a second clay event after his triumph in Rio de Janeiro this year were bolstered on Tuesday when Australian Open champion Stanislas Wawrinka lost and Roger Federer withdrew for the birth of his second set of twins. Second-ranked Novak Djokovic was absent to injury.
"I didn't even get to the necessary round to face them at the last tournaments," said Nadal, dismissing his chances despite the marquee absentees. "Every rival is difficult."
Nadal faces Jarrko Nieminen next after the Finn beat Dutch qualifier Igor Sijsling 6-3, 6-2.
Also, Tomas Berdych moved to a perfect 11-0 against Kevin Anderson following his 6-1, 6-4 win over the South African.
"I always like to play here. It's a bit faster because of the altitude, which suits my game well and means I can serve well and play aggressively," the sixth-seeded Czech said. "There was also the aspect of the many matches I've played with Kevin and all of them went my way."
In other women's results, fourth-seeded Simona Halep of Romania advanced, while Roberta Vinci of Italy knocked out Caroline Wozniacki 6-3, 6-3.
Andy Murray returns to the clay for the first time in nearly one year later Wednesday against Nicolas Almagro. David Ferrer, the next highest remaining seed after Nadal at No. 5, faces John Isner.
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