Spieth, who's attempting to complete the career Grand Slam this weekend, moved within striking distance of the leaders with four birdies on a bogey-free front nine. The triple followed by a bogey on the 13th dealt a major blow to his hopes of winning the season's final major tournament.
He stands at four under for the event, nine strokes behind leader Brooks Koepka with play ongoing.
Spieth caught fire with four birdies over his final five holes on the front nine. The moving-day surge pushed him inside the top 10 and within three strokes of the leaders.
The 25-year-old Texas native, who's winless so far in 2018, has showcased that type of form for stretches throughout the season, but he's struggled to maintain it. That's why he's tallied more missed cuts (three) than top-10 finishes (one) since his third-place showing at The Masters in April.
His inconsistency reared its ugly head once again after the turn.
He made a mess of the 12th hole following a wayward tee shot. After hacking out all over the grounds, he finally found the green but could only muster a two-putt for a triple.
Spieth followed it with another poor tee shot on the 13th en route to another dropped shot. In all, a 20-minute stretch likely cost him the tournament.
The three-time major champion's round did feature some highlights, though:
It's hard to imagine Spieth coming back to win the PGA Championship unless Koepka's game falls apart and the scoring conditions get far more difficult Sunday. Bellerive has been providing too many scoring opportunities to make up what could be a double-digit deficit.
Eliminating the poor, tournament-altering stretches like he had starting on the 12th will be essential as he starts looking toward the FedEx Cup Playoffs.