Surrey 150 for 9 (Pietersen 52, Walter 3-24) beat Essex 140 for 7 by 10 runs
Kevin Pietersen hit four sixes in an over as he made an explosive return to domestic cricket for Surrey in the NatWest T20 Blast.
The 37-year-old's 52 off 35 balls - his highest score in 22 games for Surrey in the format - helped his side to victory by ten runs, their third win in five South Group games in front of a 23,508 crowd at the Kia Oval.
Defending a total of 150 for 9 on a stodgy, two-paced pitch, Surrey restricted Essex to 140 for 7, although Pietersen was unable to field during because of a calf strain. Surrey said it was too early to assess whether he would recover in time for Friday's game against Middlesex.
Pietersen was playing his first domestic match for nearly two years and aiming to improve on a modest competition average for Surrey of 22.30. That looked unlikely in the 10th over when he picked out deep mid-wicket with a slog sweep but Dan Lawrence dropped a straightforward catch, to the obvious frustration of the unlucky bowler Ashar Zaidi, when Pietersen had made 12.
Earlier, Pietersen reverse-hit his fifth ball from Simon Harmer for his first boundary but it was in the South African off-spinner's third over that he made Essex pay for dropping him, hitting the first two deliveries for six into the crowd at long on and repeating the shots off the final two balls of an over which cost 26 runs and forced Harmer, the leading wicket-taker in the County Championship this season, out of the attack.
Pietersen added 43 with Dominic Sibley in the most productive stand of the Surrey innings and he also launched Essex captain Ryan ten Doeschate back over his head for his fifth six before holing out to long on off Paul Walter in the 16th over, three balls after reaching his 40th half-century in T20.
Finch made 21 in an opening stand of 40 in four overs with England's Jason Roy, but both fell victim to Mohammad Amir who was the pick of a disciplined Essex attack with 2 for 13 while Walter picked up three wickets in 12 balls to finish with 3 for 24.
Essex began their chase confidently, but after openers Varun Chopra (27) and Dan Lawrence (24) were parted in the seventh over with the score on 47 no Essex batsman was able to play with the freedom Pietersen had shown earlier.
Surrey captain Gareth Batty picked up Tom Westley (17), courtesy of a smart stumping by Kumar Sangakkara, who was keeping wicket for the first time in England since 2014, and Adam Wheater off successive balls and when Jade Dernbach had Ravi Bopara held at long off for 24 it left ten Doeschate too much to do.
He struck boundaries off Jade Dernbach to leave Essex needing 19 to win off the final over but the impressive Tom Curran held his nerve and conceded just eight runs.