Murray's win over the big-serving Spaniard was his 14th in succession against southpaws, part of a prolonged mastery dating back to hitting practice with his left-handed brother Jamie.
Doubles specialist Jamie Murray has had a tough tournament, complaining of heat sickness after his first round win with partner John Peers during a Melbourne scorcher before being bundled out in the second round.
The 28-year-old Briton can at least take some credit for his little brother's success against lefties, however.
"I grew up playing with my brother, he was a lefty, so I got a lot of practice against left-handed serves," Wimbledon champion Murray said in a courtside interview at Hisense Arena.
Murray, whose outstanding backhand has given him an edge against the forehands of lefties, holds a less attractive record against the best southpaw in the game, however.
"I don't have a great record against one Spanish. (He's) called Nadal or something," the fourth seed added.
Murray has won his last two matches against world number one Rafa Nadal, but is on the wrong side of a 13-5 head-to-head career record against the 27-year-old Mallorcan and trails 6-2 in Grand Slams.
The pair could clash again in the semi-finals at Melbourne Park.
After undergoing back surgery late last year, Murray has eased himself in to the year's first Grand Slam, where he has tasted heartbreak in three losing finals.
On Saturday, however, the Scot banished any remaining doubts about his physical condition by clinically dismantling the 26th seed to set up a virtual walkover against Frenchman Stephane Robert, who won a battle of lucky losers in the third round.
The 33-year-old Robert is ranked 119th in the world, made the main draw only after German Philipp Kohlschreiber pulled out and faces a monumental task to prevent Murray from extending his winning streak over French players to 14.
Against Lopez, Murray was broken in the opening game and was at his muttering, tormented best, berating himself in the opening two sets when he struggled at times with his usually dependable backhand.
However, on surrendering the second set, Lopez's resolve crumbled, allowing the 26-year-old Scot to coast to an encouraging victory.
Murray winced in discomfort at times, and spent time diligently stretching after the match was won.
"It's pretty good so far," Murray said of his back. "I mean, I've recovered well after the matches. From time to time (there's) a bit of stiffness and soreness, but for the most part it's been good.
"You normally are going to get some stiffness and stuff in an area where you've had surgery. It takes a bit of time for that to go away."
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