In this Sept. 24, 2017, file photo, A.J. Allmendinger is introduced prior to the NASCAR Cup Series 300 auto race at New Hampshire Motor Speedway in Loudon, N.H. Allmendinger knows his only chance to make NASCAR's playoffs is to win a race, and the road course at Watkins Glen gives him his best shot.

WATKINS GLEN, N.Y. — AJ Allmendinger isn’t exactly sure how it happened. More than a month later he’s still beating himself up over it, though.

“I just missed a shift,” Allmendinger said Friday at Watkins Glen International. “It’s something I’ve never really done before. It’s still on my mind, for sure.”

That critical mistake happened in late June at Sonoma, one of two road courses NASCAR’s Cup series competes on every year, and it hurt. Allmendinger, one of the top road racers in NASCAR, led qualifying after the first round and ended up fifth behind Kyle Larson, then won the first stage of the race. The missed shift came about a third of the way through the 110-lap event and blew the engine on Allmendinger’s No. 47 JTG Daugherty Racing Chevy, ending his day.

“They’re the easiest things I’ve ever shifted in my life. For the thousands of times that I’ve shifted like that, it was something I’ve never really thought of,” Allmendinger said. “Probably a little lackadaisical on second to third because it’s so natural.”

Allmendinger has a chance to atone for that mistake Sunday at The Glen. He’s high on the list of favorites because of his prowess on the twisting tracks, honed during his days in open-wheel competition. The one and only win of his Cup career came four years ago at Watkins Glen in his 213th start in the series when he held off Marcos Ambrose in a fender-bending duel at the end .

Allmendinger beat the best in Ambrose, who was dominant in NASCAR’s top two series at The Glen, and that victory made JTG Daugherty Racing eligible for the postseason.

Time to duplicate that feat because a win is the only way the No. 47 will qualify for the playoffs.

Only five races remain in the Cup regular season.