Djokovic recovered from a 3-0 deficit in the deciding set on Wednesday and questions were again tabled about the Serb's deteriorating confidence levels.
Meanwhile, Nick Kyrgios could face Rafael Nadal in a blockbuster third-round clash on Thursday after easily seeing off Ryan Harrison 6-3, 6-3.
"I played really well tonight".
Lopez argued to French umpire Damien Dumusois that Djokovic's apparent stoppage caused Lopez to not carry on playing the point but Dumusois disagreed saying that Djokovic had not called to the umpire saying the shot was long.
In his first match since changing his coaching team in an attempt to change fortunes following a hard start to the year, Djokovic was taken to three sets by Spains Nicolas Almagro in a match which lasted for two and a quarter hours.
The victory was the Canadian's second win in four matches against the 26th-ranked Muller.
The Spaniard had to save two break points to hold for 4-3 and then took his chance to break with Djokovic serving to stay in the set.
"Four consecutive games lost in this level should not happen because otherwise you're going to lose the match", added Djokovic.
The defending champion heads into the match after a hard-fought win over unseeded Nicolas Almagro in his opening encounter yesterday.
Novak Djokovic moved into the Madrid Masters quarter-finals in relative ease defeating Felicano Lopez but there was one moment of controversy in the first set. But, as I said, it was quite a solid performance from both players, high quality.
But I'm sure it's split a lot of opinion.
Murray won his only title of the year in Dubai and had started the clay season trying to regain his form following a right elbow injury that kept him out of the Miami Open and the Davis Cup quarterfinals.
Nadal had a tough start against Fognini, needing to save 10 of 12 break points conceded to the Italian in the first set alone.
More Almagro errors flowed to give Djokovic another break for 6-5 and he served out for the match. "In the end I played as I had dreamed", said Coric. "So I will try to prepare well, and hopefully I can win the match".
With a potential quarter-final meeting against sixth seed Kei Nishikori awaiting him, Djokovic would not want another draining battle in today's match.
A finalist in Madrid in 2014, Nishikori is playing in his first tournament since March. "But I can feel it a little bit", she said. "I was enjoying very much".
Quick Read: Stamp Out Hunger on Saturday, May 13
In last year's drive, nearly 78,000 pounds of food was collected in Sioux Falls and the goal this year is to reach 85,000 pounds. Fincher said food from the Shoals and Sand Mountain area will go to the food bank's partner feeding programs.
Physician dragged off flight settles with United
In a phone interview with The Associated Press, Demetrio said the settlement also averts any lawsuit against Chicago officials. Demetrio said his client became an "unintended champion" of the changes that will improve the lives of millions of travelers.
Police officer who shot and killed Texas teen is fired
The teenager's brothers "witnessed firsthand" what happened, Merritt said. "Their young lives will be forever altered". Last year, Tulsa, Oklahoma police officer Betty Shelby shot and killed Terence Crutcher in the middle of a roadway.