ENGLAND (AFP) — Rory McIlroy welcomed overnight changes made to bunkers at the British Open on Friday after players complained they were too tough.

Masters champion Jon Rahm described Royal Liverpool's 82 bunkers as "proper penalty structures" after having to play backwards out of one during his opening 74 on Thursday.

McIlroy needed two shots to escape a greenside trap on the 18th while two-time US PGA winner Justin Thomas had a nightmare on the final hole of his first round at Hoylake after finding a bunker, ending up with a quadruple-bogey nine.

Former British Open champion Stewart Cink described the bunkers as "very penal."

"Eventually it'll catch up with you," the 2009 winner said. "The bottoms of them are so flat that if a ball comes in with any momentum, it's just going right up to the lip and stop."

In response, tournament organizers the R&A instructed greenkeeping staff to build up the edges of the bunkers to allow more balls to roll back into the center.

"Yesterday afternoon (Thursday) the bunkers dried out more than we have seen in recent weeks and that led to more balls running straight up against the face than we would normally expect," the R&A said in a statement.

"We have therefore raked all of the bunkers slightly differently to take the sand up one revet on the face of the bunkers."

McIlroy, who finished his second round at one under par, nine shots behind leader Brian Harman, said he noticed the difference after making a birdie at the fifth despite finding the bunker.

"I hit a four-iron into the fifth hole today and it pitched on top of the bunker and came back in," he said.

"I didn't know at this point that they'd made that little gradual rise up into the face and when I got up there, I was pleasantly surprised that I had a shot.

"I wouldn't say there's one person in the field that wouldn't welcome that change."

American Brian Harman stormed into a five-shot lead at the British Open on Friday after a faultless 65 left him on 10 under par as bigger names struggled to keep pace.