The 22-year-old Spaniard won for the 52nd time this season by seizing control after struggling to capture the first set against his 27th-seeded German foe.
"I was a little bit nervous because for me it was important to finish the match," Nadal said. "And, well, lucky for me later I played a good game (to win the match)."
Also Saturday, eighth-seeded Richard Gasquet topped Gilles Simon 6-3, 6-3, 3-6 (3-7), 6-3; No. 17 Mikhail Youzhny of Russia topped No. 16 Radek Stepanek of the Czech Republic 5-7, 7-6 (7-5), 4-6, 7-6 (7-4), 6-3; and unseeded Marin Cilic of Croatia knocked off No. 14 seed Paul-Henri Mathieu 6-7 (5-7), 6-3, 6-4, 7-6 (8-6).
Other winners were 12th-seeded Andy Murray, Rainer Schuettler, Arnaud Clement and the 25th seed, Dmitry Tursunov of Russia.