“Obviously,’’ said the blue-eyed, sweet-swinging Mickelson, “it was a lucky shot.’’

Almost 20 years into his pro career, and more than $63 million in career earnings jam-packed into his bag of clubs, Mickelson’s definition of luck is not the same as yours, dear reader. There is a PGA pro’s luck, not to be confused in any sense or fashion with everyman, golf hacker’s holy-smokers-did-you-see-that!? kind of luck.

In fact, the Official Hacker’s Golf Guide of Good Fortune (way out of print) includes such serendipity as:

■No one needs to visit the local ER during your foursome’s first 15 holes (if later, hey, at least it didn’t rain).

■The tick bite does not signal the onset of Lyme disease, though that cycle of antibiotics is always advised.

■When the cart catches fire, you still have plenty of cheap beer in the cooler to quench the flames and never have to break into the Canadian stuff, eh?

■No one is home, or at least awake, when your tee shot rockets through the picture window of the stately Colonial that is within range at No. 9.

■The course ranger, fuming mad at your six-hour pace of play, packs it in after 11 in order to make the early-bird special at Olive Garden.

Nope. The likes of Mickelson and all his PGA brethren who have played across TPC Boston’s emerald lawn these last three days don’t even think of factoring luck into their day-to-day workplace equation. Like the Tour’s slogan has reminded us for years, these guys are good, really good, and Lady Luck can take a hike when they have a club in their hands.

But Mickelson’s astounding shot at the 459-yard No. 12, he said, fit into a pro’s definition of luck. For starters, he sliced his drive left, way left, rolling to a stop 286 yards from the box.

“It was the one bad drive I hit,’’ he said.

Yes, one. All day, a single bad shot off the tee. The rest of the lot, 94.5 percent, were acceptable or better by Mickelson standards. That’s just not right, is it? Hackers, we hear your groaning, your slaps to the forehead.

“It was the one time I left the drive out a little bit left,’’ continued Mickelson, who entered Week No. 1 of the FedEx Cup playoffs parked in 11th place for the $10 million top prize, after dropping five slots the prior week. “And it was in the thick rough.’’

Summon Lefty’s luck. That itsy-bitsy cup was way out there, 519 feet to be exact. (Note: Hackers measure in feet, because anything in the 400-foot ballpark must be considered a potential putt.) Even the pros don’t figure they are going to spike 519-foot shots into a hole that is 4 1/4 inches in diameter.

Mickelson took aim, fired, waited. He knew it was going to hook. When the crowd erupted, his eagle was official, his card reading 7 under with six holes to go.

“I was just trying to get it to the middle of the green,’’ he said. “With the stance I had, it was going to hook quite a bit. It hooked a little bit more [than expected], and it went in. I mean, that was fun to see.’’

These are the shots that try hackers’ soles (those with rubber cleats) and souls (those with ever-resilient hearts). For guys on Tour, these shots are the instruments of wonder that attract big crowds to rolling cornfields around America and keep eyeballs returning to network TV, although not as many as when He Who Will Not Be Named (absent from Norton this week) is teeing it up.

Mickelson has known bad luck, too, of course. What golfer hasn’t? His play on No. 18 at the 2006 US Open at Winged Foot is legendary, a slice of hackdom to be remembered in perpetuity. He put a shot into a garbage can on No. 17, somehow managed par, and only needed to break even on the final hole to capture the win. He went left with his drive on 18, a wild screamer that dinged off a corporate tent (cheer here, hackers). Rather than play a safe shot back to the fairway with his follow-up, he chose bold over boring, only to crank the ball off a tree for a net result of about a 50-yard advance.

When the whole bit of folly and mayhem came to an end, Mickelson had a double bogey and finished tied for second. Geoff Ogilvy walked off the winner, with a humbled Mickelson left to mutter, “I am such an idiot.’’

No such talk yesterday. Luck lifts both a man’s spirits and his IQ.

“I’ll get some times where it will hit a tree and go back into the fairway,’’ Mickelson mused, asked to define better what he considers luck. “That seems to happen once a round, so there you go.’’

The media listening to Mickelson, no doubt hackers all, provided knowing chuckles over the words, “so there you go.’’

“You hit enough in there,’’ he continued, “eventually one will kick out. But that shot [on No. 12], to go in the hole obviously was extremely lucky . . . to pull it just the right amount was very lucky.’’

Mickelson finished out his bogey-free round with another birdie on 18, for an 8-under 63, a huge improvement over his 70-73 start. He was so focused on switching to a belly putter here, he only practiced on the green for four days leading up to Friday’s start. On the third day, he had his swing fully reengaged, and who knows what that might mean for today’s finale (he is parked in 19th after 54 holes). For the record, Mickelson has won the last six tournaments in which he has posted a round of 63 or better, including this year’s Houston Open, when he fired a 63 in Round 3. The last time he shot a 63 and didn’t win: the 2003 Bob Hope Classic, tied for sixth.

“It was a really good ball-striking round,’’ he said.

With a little bit of luck in the sidecar. So there you go.

Kevin Paul Dupont can be reached at dupont@globe.com.