"That rough is just mental," declared Forrest after staging a pleasing recovery to card a one-over-par 73 on a day when a toughened up course, coupled with a gusting strong wind, sent scores soaring at a venue where American Bryson DeChambeau shot the lights out 12 months ago to triumph with a 24-under-par total.
"I didn't really hit that bad a drive off the tenth - my opening hole - but I had a terrible lie," added the 26-year-old, who sits as the leading Scot along with two-time winner Stephen Gallacher and Richie Ramsay, the trio lying six shots behind the pacesetter, Belgian Thomas Pieters.
"I tried to be sensible by going with a 9-iron, but it only went 10 yards. I had a better lie for the next one and thought I could get a rescue on it, but it went 20 yards. I got the sand wedge on the next one, wedged it on with my fifth shot and two-putted and I thought to myself, 'blimey, this is going to be a long day!'"
Scoring here has traditionally been low. Before DeChambeau ripped the Karl Litten-designed course apart with rounds of 66-66-68-64, China's Haotong Li won with a 23-under total in 2018, while Stephen Gallacher and Rory McIlroy both shot 22-under in getting their hands on the iconic coffee pot.
With some of the fairways having been narrowed, the greens being firm and that rough being thick, though, it seems as though the record high of 11-under, which did the trick for Spanish duo Miguel Angel Jimenez and Alvaro Quiros in 2010 and 2011 respectively, is more likely this time around in the $3.5 million event.
"It is probably the toughest test I've faced as a professional," admitted Forrest. "I've not played a US Open yet, but some of the guys are saying the rough in that at some of them in the past is not as bad as here. This and Leopard Creek are probably the two toughest courses I've played since I came out here."
Prior to the wind dying down later in the day, when Pieters came in with his effort to lead by a shot from American David Lipsky, only 10 out of the 66 players in the morning wave bettered the par of 72.
In his first appearance here since 2004, Padraig Harrington was one of them with a 71. The Ryder Cup captain, one of the most experienced campaigners around, agreed that the test this week is tougher than people have come to expect here, but he doesn't feel it has anything to do with Rory McIlroy claiming late last year that European Tour set ups were not tough enough.
"I don't see it as a reaction to that. I see it was a reaction that this course played easy last year," said the Dubliner. "Twenty-four-under par won it last year and they came out all guns blazing. The fairways are narrow. They are winding. They are very difficult to hit.
"Last year, the greens were soft, but this year they are fast, seriously fast. I can't remember playing greens with a combination of firmness and fast and that speed. Firm, fast greens just sort us out, especially if you throw a little bit of wind in.
"The rough's not actually the biggest trouble out there. It's the actual firmness of the greens and trying to control the ball if you're out of position."
Helped by the only birdie at the difficult par-4 ninth hole among the morning starts, DeChambeau opened with a two-under 70 to sit handily-placed in joint-15th. “It was just so difficult out there today and just a completely different challenge from a year ago,” said the American.
“If the conditions stay the same, I would say the course is going to play four shots tougher each of the four days compared to a year ago. I felt like that even par last year was around three to four-under and today I felt like I shot four or five-under even though it was only two-under."
Pieters, a four-time European Tour winner, carded an impressive eight birdies in the tough conditions. He was out in five-under 32 before recovering from a shaky start to his back nine with three birdies in a row from the fifth.
“There was nothing easy about today, but I found 14 greens, which is always nice in these kind of conditions," said the leader. “I've been working on my putting real hard, and I just hope to see some progress. I saw that today."
Lee Westwood, last week's winner in Abu Dhabi, was running on empty as he opened with a 78.
"I'm absolutely knackered. I've hit a wall," said the Englishman. "I could have done with nine hours sleep last night and got three. Me being 46 and not 26 showed up right there."
Gallacher, the winner here in 2013 and 2014, recovered from a bogey-bogey start to card his 73, matching Ramsay's effort earlier in the day.
In his first event of the 2020 campaign, last year's Rookie of the Year Bob MacIntyre had picked up three shots in five holes before finishing with a bogey for a 74, the same score as both David Drysdale and Scott Jamieson.
After being two-under early on, David Law had to settle for a 75, two shots better than his mentor and manager Paul Lawrie on a day when former Scottish Stroke Play champion Andy Sullivan was disqualified for signing for the wrong scores at two holes.