Returning to the tournament where he's had a decade worth of success and become a de facto ambassador, Zach Johnson was asked what aspect of his game he's been working on in advance of this week's John Deere Classic.

"I wouldn't even know where to start on that one," Johnson told reporters Tuesday.

It's been a lean year for the two-time major champ. Johnson tied for seventh at the RSM Classic in November, but that remains his most recent top-10 finish on Tour. He's missed 4 of 14 cuts since the calendar flipped to 2019, with a T-16 result at the RBC Heritage in April the only time he cracked the top 20. It's all led to a steady decline in the world ranking for the typically consistent veteran, who last week dropped out of the top 100 in the world for the first time since 2004.

Often one of the highest-ranked players in the Quad Cities each year, Johnson is now ranked No. 108 - a fall of more than 40 spots since January. With just four weeks left in the regular season, he's 140th in FedExCup points, in danger of missing the 125-man playoffs for the first time since their advent in 2007.

"There's been a lot of frustration for me in the 2019 season because of the work I've put in and not really seeing the results," Johnson said. "Mentally I haven't been quite as sharp, and then my practice probably hasn't been as strategic as it needs to be."

Johnson described his recent practice as "more analytical and assessing" than focused on improvement, leading to periods of time and sweat that ultimately didn't advance his game. But he regrouped with his team ahead of the U.S. Open, where he finished T-58, and returns to TPC Deere Run rested and ready to pounce on one of his favorite venues.

Johnson's Deere record is nothing short of extraordinary: a win in 2012, three runner-up finishes and seven top-5s in a nine-year span. Dating back to a T-2 finish in 2009, he has averaged better than 4 under par for each of his 40 competitive rounds. From 2012-15, he had only one score higher than a 68 while finishing no worse than a tie for third.

A native of Iowa making his 18th tournament appearance, Johnson will need all the good vibes to correct a tailspin that has been months in the making. But less than four years removed from his Open triumph at St. Andrews, he remains optimistic that there are still plenty of low rounds left in the bag.

"I still feel that my best golf is in front of me. I don't know how else to word that," Johnson said. "I mean, I'm 43. I know, again, the realist in me understands that there's probably things that, regardless of how good I'm swinging it, how good I'm hitting it down my line, age can be a factor there. But I'm not going to use that as an excuse. Just not going to happen."