Haffenden's 61 not out from 60 balls meant Sidley triumphed by three wickets with nine balls to spare in a Sussex Cricket League Division 10 East derby away to Bexhill Strikers.
Chasing a target of 204 in the sunshine at Bexhill Down, Sidley looked to be in a spot of bother after subsiding from 71-1 to 95-6.
But a vital seventh-wicket partnership of 89 between Haffenden and Sabbir Ahmed (40) got the visitors back on track before Louis Haffenden joined his dad Johnathan to see Sidley over the winning line.
Earlier in the innings, Moinul Islam, making just his third Sidley appearance, smashed 42 off 32 deliveries and shared a second-wicket stand of 69 with Asanka Pathiranage (21). Sandeep Satheesh was the pick of the Strikers bowlers with 3-32.
Sidley prevailed despite Jayan Kalathingal's big-hitting century during Strikers' total of 203 all out after they'd chosen to bat on winning the toss.
The number five bludgeoned 102 from just 80 balls with six fours and seven sixes to help the hosts recover from 95-6 and 122-7 to post a very competitive opening-day score.
Louis Haffenden was Sidley's most successful bowler with 3-45, Abdullah Al Arif took 2-14 on his league debut for the club and captain Steve Ramsden claimed 2-43.
Sidley lost both meetings with Strikers last season, when both teams won promotion from Division 11 East (South), with Strikers going through the entire campaign undefeated.