The hardest thing to do in golf -- and maybe sports -- is close out a tournament you're defending with a slim lead going into the final day. That's exactly what Paul Casey did on Sunday at the Valspar Championship as he held off Dustin Johnson, Louis Oosthuizen and Bubba Watson for his third PGA Tour victory. 

Maybe most impressively, Casey did it with the No. 1 player on the planet on his hip. Johnson trailed by one entering the final round, but Casey birdied the first and never took on a serious threat from the most threatening man in golf. It was Casey's sixth 54-hole lead but just his second victory in such scenarios.

Following a hilly front nine of 1-over 37, Casey tightened his game and his lead on the field with birdies on two of his first five holes on the back nine, and he played the final three holes -- known for being named after a serpent at Innisbrook Resort -- in 1 over. The 72 in Round 4 was better than the field average on Sunday, and enough to seal the deal by one over Oosthuizen (and late closer Jason Kokrak). 

It was the same margin he won by last year over Patrick Reed and Tiger Woods, but it wasn't even close to the same feeling, Casey said.

"Today wasn't easy," Casey told NBC Sports. "That felt very different since winning last year. Last year's win was so big. That felt like my first victory as a professional. I've felt so different since then. New confidence. I'm getting older, but I feel like I'm getting better ... This is so cool, I'm so happy."

Now the Englishman will go to Augusta with even more of that confidence following a spring in which he has a pair of top-five finishes and a victory on the PGA Tour. He's ... well, he's a sneaky fun pick to win his first major three weeks from today.

Regardless of whether that happens, Casey does seem to be getting better with age. The 41-year-old won just once in his first 224 PGA Tour starts, but he's now won twice in his last 24 -- this year over the best player alive, last year over the best player in history. It's been a pretty grand two-year stretch. Grade: A+

Here are the rest of our grades for the 2019 Valspar Championship.

Dustin Johnson (T6): D.J.'s streak of rounds in the 60s came to a surprising end on Sunday in the final pairing with Casey. He made no birdies, less than 50 feet of putts (he made 90 in each of the first three days here), and for the second Sunday in a row, Johnson lost strokes to the field on the greens (he's missed three putts from under 5 feet in the last two weeks). It was about as uninspiring a performance as I've ever seen from the No. 1 player in the world. Better to get it out of his system now than in three weeks, I suppose. Grade: A-

Jon Rahm (T6): It was a good bounce back for Rahm who both melted down at the Players last week and affirmed earlier this week that he's not going to change who he is just to placate everyone (which is great!). Rahm really only had one poor nine-hole run (on Saturday) and finished top 20 from tee to green. He's currently in the category of a handful of players who will almost certainly contend if they're in the field. It's an impressive (and under-appreciated) quality. Grade: A

Patrick Reed (MC): The reigning Masters champ lost seven (!!) strokes to the field from tee to green en route to a 77-75 missed cut. He shot nine-hole scores of 35, 38, 39 and 40, made five double bogeys in 36 holes and asked swing coach David Leadbetter to take a gander at his swing (or, more accurately, had his wife Justine ask swing coach David Leadbetter to take a gander at his swing). Other than that, things are really shaping up for Augusta in two weeks. Grade: F