The 13 most difficult holes on the PGA Tour in the 2016-17 regular season

While PGA Tour players make most golf holes look easy, believe it or not there are plenty of holes out there where the field in a given week struggles mightily.

Two key factors come into play: the weather and the severity of the course setup/course condition.

Both were present at the PGA Championship in August, which is why it's no surprise that four holes at Quail Hollow counted among the 13 most difficult on the PGA Tour in the 2017 regular season.

RELATED: Most difficult courses on Tour in 2017 | Best major moments in 2017

Thirteen — notoriously unlucky — felt like the right number to use for a list detailing the nightmares for the world's best players. Here's the list:

13. No. 2 at Torrey Pines North, a 495-yard par 4.

One of two courses used for the Farmers Insurance Open, the hole played to an average score of 4.342.

Eagles: 0
Birdies: 5
Pars: 94
Bogeys: 54
Double bogeys: 2
Other: 0

12. No. 12 at Quail Hollow Club, a 456-yard par 4.

Home to the 2017 PGA Championship, this is the first of four holes at Quail Hollow on this list. The hole played to an average score of 4.346.

Eagles: 1
Birdies: 41
Pars: 243
Bogeys: 146
Double bogeys: 28
Other: 0

11. No. 8 at Club de Golf Chapultepec, a 525-yard par 4.

This hole at the host venue for the WGC-Mexico Championship played to a scoring average of 4.359.

Eagles: 0
Birdies: 24
Pars: 163
Bogeys: 104
Double bogeys: 10
Other: 3

10. No. 16 at Quail Hollow Club, a 525-yard par 4.

During the PGA Championship, this hole had a scoring average of 4.362.

Eagles: 0
Birdies: 25
Pars: 275
Bogeys: 130
Double bogeys: 26
Other: 3

9. No. 16 at Muirfield Village GC, a 201-yard par 3.

This difficult par 3 at the Memorial Tournament in Dublin, Ohio, played to a scoring average of 3.367.

Eagles: 0
Birdies: 26
Pars: 228
Bogeys: 107
Double bogeys: 25
Other: 4

8. No. 11 at Augusta National Golf Club, a 505-yard par 4.

Annually one of the toughest holes at the Masters, players face a tight tee shot with everything feeding left. On the approach, left is dead into the creek, while bailing out right leaves a daunting pitch back toward the water. In 2017, the hole played to a scoring average of 4.373.

Eagles: 0
Birdies: 14
Pars: 170
Bogeys: 96
Double bogeys: 11
Other: 1

7. No. 1 at Quail Hollow Club, a 524-yard par 4.

This sweeping dogleg right plays as a par 5 for members, but was a long par 4 during the PGA Championship. Anything right off the tee was pretty much dead. And with all the troublesome bunkers surrounding the green, players needed pinpoint accuracy to get on this green in regulation. It played to an average score of 4.388.

Eagles: 0
Birdies: 27
Pars: 240
Bogeys: 179
Double bogeys: 13
Other: 0

6. No. 18 at Quail Hollow Club, a 494-yard par 4.

The final hole of the intimidating Green Mile, this tight hole has trees up the right side with bunkers and a creek up the left side that eventually protects the green. It played to an average of 4.414 during the PGA Championship.

Eagles: 0
Birdies: 30
Pars: 255
Bogeys: 135
Double bogeys: 33
Other: 6

5. No. 6 at Royal Birkdale, a 499-yard par 4.

The hole played to a scoring average of 4.427 during the 2017 Open Championship.

Eagles: 0
Birdies: 26
Pars: 245
Bogeys: 173
Double bogeys: 17
Other: 5

4. No. 12 at Torrey Pines South, a 504-yard par 4.

During the 2017 Farmers Insurance Open, this scenic hole played to a 4.435 scoring average.

Eagles: 0
Birdies: 12
Pars: 167
Bogeys: 113
Double bogeys: 15
Other: 1

3. No. 1 at Augusta National Golf Club, a 445-yard par 4.

The opening hole at the Masters is arguably the toughest start in golf. If your tee shot isn't both long and good, you're going to have a blind approach to a severe green. During the 2017 Masters, it played to a scoring average of 4.462. In the 2016 Masters, Ernie Els famously made a 9 on the hole in the first round.

Eagles: 0
Birdies: 18
Pars: 152
Bogeys: 99
Double bogeys: 16
Other: 7

2. No. 18, TPC Sawgrass, a 462-yard par 4.

With water all the way up the left side of the hole, this is a brutal closing hole. It's all about being in the proper position off the tee. In the 2017 Players Championship, the hole had a 4.486 scoring average.

Eagles: 0
Birdies: 37
Pars: 226
Bogeys: 120
Double bogeys: 45
Other: 12

1. No. 11, a 470-yard par 4 at TPC Potomac at Avenel Farm.

Used during the 2017 Quicken Loans National, this beast played to a scoring average that was more than a half-stroke over par at 4.521.

Eagles: 0
Birdies: 23
Pars: 189
Bogeys: 132
Double bogeys: 27
Other: 11

In case you're interested, you have to go all the way down to No. 135 on the list to find the first par-5 hole. That would be the 603-yard eighth hole at Sheshan International GC, home to the 2017 WGC-HSBC Champions. The hole had a scoring average of 5.173 and didn't cough up a single eagle. There were 51 birdies, 185 pars, 54 bogeys, 16 double bogeys and 6 others for the week. Note: There were only 78 players in the field.