Busch furiously tried to catch him as the two weaved their way through lapped traffic, but Harvick held firm in the No.
Only six cars finished on the lead lap in a tepid event at the 0.533-mile bullring revered for bumping and banging.
Track officials sold all 30,000 tickets allowed — making Bristol one of the largest sporting events since the pandemic — and the spectators crowded the fence as Harvick celebrated his second win of the playoffs.
Erik Jones, who is not eligible for the playoffs, finished third for Joe Gibbs Racing and was followed by rookie Tyler Reddick and playoff contenders Aric Almirola and Clint Bowyer.
Denny Hamlin, considered the closest challenger to Harvick this year, closed out a sub-par opening round of the playoffs with a 21st-place finish.