Collin Morikawa joked he became a spectator as the pace of play proved an issue on the first day of the Open at St Andrews.

Some of Thursday's rounds took longer than six hours as the layout of the Old Course, with its double greens, joint fairways and numerous crossover points, slowed the 156-man field.

The slow rounds meant that the late-starters only concluded their rounds in near darkness. Ireland's Ronan Mullarney, in the final group, holed out on the 18th at 10.25pm.

Defending champion Morikawa, who was in the 19th of the 52 three-ball groupings alongside Xander Schauffele and Rory McIlroy, admitted it had been frustrating.

"I figured it would be slow, but I didn't know it would be this slow," said the American, winner at St George's last year, after his opening level-par 72.

"We were waiting on groups at tees, waiting on fairways. Xander and I talked about it, we're watching more golf than we ever have.

"You stay in the fairway and you're watching two other groups play golf."

McIlroy did not let the matter bother him too much as he carded a six-under-par 66.

The 2014 champion said: "It's quite stop-start but I think St Andrews is that way.

"There's a lot of criss-crossing and waiting on other greens and waiting on greens to clear because they're drivable par fours.

"So I think, especially the first two days when it's the full field, it's to be expected. It is what it is."