Lopez, ranked a career-high 12th in the world and the top seed for the tournament, was a little sluggish at times but he managed to blunt Groth’s booming serves to book a quarter-final clash with eighth-seed Sam Querrey.
“I’m very pleased to be here again,” said the 33-year-old.
“I need a few more matches to start moving better and hitting the ball cleaner, but it was very difficult because he was serving huge. I think I did quite good overall.”
Querrey advanced with a 6-1 4-6 6-4 victory over fellow American and friend Steve Johnson.
“It’s not fun,” said Querrey of playing a pal. “I’m definitely glad I won, but it doesn’t quite feel the same.”
Third-seeded South African Kevin Anderson, who like Lopez had a first-round bye, defeated Argentina’s Federico Delbonis 6-7 (4-7) 6-0 6-3.
“I felt I was moving quite well,” Anderson said. “There were a few sort of hesitancies as far as where I was on clay, and I think I far exceeded that. He was a very tough opponent, so I had my work cut out for me today.”
Anderson next faces seventh-seeded Jeremy Chardy of France, a 6-3 6-3 winner over Japan’s Go Soeda.
“He’s a very talented guy,” Anderson said of Chardy. “We’ve played doubles a few times, so he’s one of my good friends on the tour. I’m going to have to play well.”