NEW YORK: Amid another difficult loss at the US Open, Andy Murray became angered when the chair umpire on Arthur Ashe Stadium stopped a point after a loud, gong-like noise.

Murray had been in control of a rally on break point at 30-40, with the score tied at 1-1 in the fourth set of his quarterfinal against Kei Nishikori.

"That's not fair," Murray said to umpire Marija Cicak. Nishikori won the replayed point, and the next seven games, on his way to a stunning 1-6, 6-4, 4-6, 6-1, 7-5 victory in 3 hours, 58 minutes.

The world No. 2 never seemed to shake off the incident, and he argued with the umpire at the change of ends after Nishikori had held serve. "On set point in the second set, there was a noise, and you said we keep playing when that happens," Murray said.

The USTA released a statement minutes after the match had ended to address the cause of the noise.

"One of the three digital audio sound processors in Arthur Ashe Stadium malfunctioned," the statement read. "The malfunctioning unit is located at the court level. The three processors are linked, and work as a single unit.

"The malfunctioning unit could not be taken off-line without interrupting play. The malfunctioning unit will be replaced between the day and evening sessions. The replacement of the unit, which requires the shutting down and then re-booting of the system, can take up to thirty minutes."

Murray had continued to complain in his chair to referee Wayne McKewen after the incident.

"Stopped the point, and I was just curious why that was and that was it," said a downbeat Murray, who came in quickly after the defeat to speak to the media.

"Wayne McKewen told me that it happened four times during the match that the speakers had gone off like that. I had only heard it one time before, which was on set point in the second set. That was it."

Murray had looked to be on course for a quick victory after taking apart Nishikori in a first set that lasted just 35 minutes. But it started to rain in the third game of the second set, and the roof was closed on Ashe.

Nishikori admitted the delay allowed him to regroup and adjust his tactical plan with coach Michael Chang, whereas the British No. 1 began to struggle in the more humid conditions.

The percentage of first serves Murray got in play dropped from 68 in the first set to 56 in the second set, and all the way down to 39 in the fourth set. Murray was broken nine times over the whole match, having broken Nishikori on eight occasions.

"It was obviously different serving under the roof," Murray said. "I started off the match serving pretty well. It slows the conditions down so it becomes easier to return. He started returning a bit better. I didn't serve so well.

"I don't think the reason I lost the match was because of my return game. I broke serve enough times. I just didn't hold serve enough. That was the difference.

"I think definitely under the roof he was able to dictate more of the points. He was playing a bit closer to the baseline than me and taking the ball on a little bit more. At times I was doing more of the running."

The roof remained closed for the rest of the match, despite sunshine and blue skies returning over Flushing Meadows.

"We were told at the beginning of the event and also today that if the forecast's good they will open the roof during the match," Murray added. "Why that didn't happen today I don't know.

"Obviously delays and stuff in matches aren't good for TV and people that are watching. You want a continuous match. You want to try and complete as many matches outdoors as possible."