The second-seeded Nadal, who is on a collision course with Roger Federer for a possible semifinal matchup, moved to 9-2 all-time against Lopez and moved to the round-of-eight Down Under for a sixth straight year.
The last time Nadal lost before the quarterfinals at a Grand Slam was at the 2009 French Open.
Lopez, seeded 18th, is the only left-handed player to have defeated Rafael Nadal on two occasions, but he has never even won a set against his Davis Cup teammate on a hard court surface. Since the start of 2010, only three different men have defeated Nadal in Grand Slam play - Novak Djokovic (twice), David Ferrer and Andy Murray (once each).
Nadal, bidding to win his second Australian Open title (2009) and his 11th Grand Slam championship, will face either No. 7 seed Tomas Berdych or No. 10 seed Nicolas Almagro in the quarterfinals. It's a good bet he'll advance past either one, as Nadal leads Berdych 10-3 and Almagro 7-0 in previous encounters.
Federer has a Sunday night matchup with rising Aussie sensation, 19-year-old Bernard Tomic.
The former top-ranked Federer boasts a men's record 16 Grand Slam titles, including a men's Open era record-tying four Aussie championships. He lost to Nadal in the 2009 title match at Melbourne Park.