Murray will have to improve his serve against Lopez in the semi-finals
Britain's Andy Murray beat Alejandro Falla at the Farmers Classic in Los Angeles, but the top seed has plenty to work on ahead of his semi-final.
Murray, playing his first event since splitting with coach Miles Maclagan, struggled with his serve throughout.
The Scot made just 42% of his first serves, well below the figure normally expected of a top player.
But Falla could not take advantage as the Scot won 7-6 (7-3) 6-1 to set up a semi-final clash with Feliciano Lopez.
Murray revealed that he felt better after his win over Falla than in the previous round when he needed three sets to beat American Tim Smyczek 6-1 4-6 6-2.
"I had the same sort of thing earlier this year after the Australian Open when I didn't play for a few weeks," he said.
"Then I played in Dubai, I was really sore after the first match, and then each match after that I started to feel a lot better. Hopefully that'll be the case here."
The first set of the quarter-final was a tough battle for the world number four, who broke to go 3-2 up only for Falla to instantly respond to recover to 3-3.
Both players had chances in the subsequent games, but the set went to a tiebreak which also went to 3-3 before Murray made his class count by winning four points in a row to take it.
Breaks were once again exchanged at the start of the second set, but Murray - despite a first-serve figure of just 38% for the set - stormed to five games in a row to end Falla's resistance.
Murray and Lopez have been practising together, and the 23-year-old said he would have his work cut out against the Spaniard when they meet in the semi-final at 0330 BST on Sunday.
"I haven't played him for a long time but we practise quite a lot and we know each other's games quite well,"
Murray told his personal website.
"It'll be a tough match."
Lopez, the world number 27, beat James Blake 3-6 7-6 (8-6) 6-4 to reach the last four.
Second seed and defending champion Sam Querrey overcame an erratic serving display to reach the semi-finals with a nail-biting 6-2 3-6 7-6 victory over Germany's Rainer Schuettler.
The American smashed two rackets in frustration and trailed 4-5 in the deciding set, but twice broke the German as his opponent served for the match before clinching the tiebreak 7-4.
Querrey will face sixth seed Janko Tipsarevic after the Serb upset third seed Cypriot Marcos Baghdatis 6-3 7-5.