The scorching heat that plagued Melbourne Park all week had given way to a pleasantly cool summer's day but the moody Briton fumed and berated himself throughout the opening two sets at Hisense Arena as he struggled with his backhand.
However, on surrendering the second set, 26th seed Lopez's resolve crumbled, allowing the 26-year-old Scot to coast to an encouraging victory in 135 minutes.
Fourth seed Murray, a three-times finalist at Melbourne Park, will next play French lucky loser Stephane Robert for a place in the quarter finals.
Caroline Wozniacki crashes out in Melbourne
Former world number one Caroline Wozniacki made an early exit from the Australian Open on Saturday with a 4-6 7-5 6-3 loss to rising Spanish talent Garbine Muguruza in the third round.
Dane Wozniacki, seeded 10th, appeared to be on course for a sixth appearance in the fourth round in seven visits to Melbourne Park when she wrapped up the first set in 39 minutes.
Venezuela-born Muguruza, however, arrived in Melbourne in good form after winning the warm-up tournament in Hobart as a qualifier and rallied to clinch the second set in just under an hour on Rod Laver Arena.
With both serves under pressure in the decider, the 20-year-old Spaniard converted all four of her break points to reach the fourth round of a grand slam for the first time.
Muguruza next meets fifth seed Agnieszka Radwanska of Poland.