Jordan Spieth never saw this coming — his worst start on the PGA Tour on one of his favorite courses.

Spieth dropped six shots over the last seven holes Thursday at Riviera, capping it off with a three-putt double bogey from 8 feet for an 8-over 79 in the Northern Trust Open. It was his worst score since an 80 in the third round of the 2014 Tour Championship.

"It's just a day to forget," Spieth said. "It's one in, hopefully, every couple years. I've shot 80 before — I've shot in the 80s a couple times on tour. I think I shot 79 today. In the course of a career, I imagine it's going to happen. Just unfortunate when it actually does."

The world's No. 1 player beat only one other guy in the 144-man field. Steven Bowditch shot an 81.

Even at the end of the round, there was still a chance his score could go up. Spieth summoned a rules official to the scoring area and asked them to review video of his third shot on the ninth hole to make sure the ball didn't move.

If it did, he would be penalized one shot under Rule 18-2.

"I don't think it moved, and they're going to be able to check with TV and let me know tomorrow," Spieth said. "I'm sure it didn't. It caught my attention because the club got a little stuck. Typically, it would do as you're setting it down. And it was oscillating as I took it back, as it (the club) got stuck."

That was the least of his concerns.

He was 16 shots behind Camilo Villegas and needing something in the low 60s to make the cut.

"I'm not throwing this tournament away," he said. "I believe that I can shoot 10-under par on this golf course. There's a 9-under round right now (until Villegas bogeyed his last hole for a 63), and I think I can do that. I'm not packing it in by any means."

Spieth was coming off a tie for 20th at the AT&T Pebble Beach Pro-Am, when the tournament got away from him with a 74 in the third round at Pebble Beach. It was the first time since September he finished out of the top 10.

He attributed that to not managing expectations.

He opened the year with an eight-shot win at Kapalua and fell into the trap of thinking it would be like every week.

This was a far cry.

He hit the ball where he wasn't supposed to, and he couldn't make anything on the greens. Scrambling typically isn't an issue, except that Spieth was leaving himself par putts in the 6-foot range on poa greens that get bumpy in the afternoon. It was a bad combination, and he paid dearly for it.

The only difference was his attitude. Spieth remained upbeat, and he was bantering with longtime friend Justin Thomas to the very end.

Thomas tapped in for par on the 18th and looked over to Fred Couples to make sure he didn't step in his line. He was nowhere near Spieth's through line, though Spieth couldn't resist saying, "Justin, you didn't ask me. You're probably in my through line at this point."

And then Spieth's par putt ran 4 feet by the hole. He missed that one for bogey and curled in this third putt for a 6.

"I felt like there was a couple rounds last week where I shot 2 under and I was more miserable than I was today," Spieth said. "It's amazing. I feel really confident about where my game is, and had a great range session. Cameron (McCormick) is here, and there was nothing pinpointed that was wrong. I just played really poorly from good positions off the tee, and didn't strike the iron shots bad either. I just overdrew it when you can't be left and I overcut it when you can't be right. Just bad timing."

Spieth said he didn't trust how Riviera was playing — slightly softer because of overnight rain — and his bad start never improved.

He missed a par putt inside 3 feet on No. 12. He took two chips to get to the green on No. 13 and scrambled for bogey. He ran a birdie putt from the fringe 5 feet by on the 15th and missed that par putt. He failed to save par from a bunker on the 16th. And then he ended with a double bogey.

"Just seemed to be anywhere you really shouldn't go, I went," he said. "And again, it's just a day to forget."