The 37-year-old former England batsman smashed five sixes,
including four in one Simon Harmer over, in his 35-ball knock as he played his
first game for Surrey in more than two years.
His effort helped Surrey to 150-9, although he was unable to
field after picking up a calf strain while batting.
Essex struggled for momentum in their reply and fell short on
140-7.
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Pietersen came to the crease when England one-day batsman Jason
Roy was out for 17 when he trod on his stumps facing Essex's Mohammad Amir,
with the Pakistan international left-arm paceman then having Aaron Finch caught
behind on his way to 2-13 from his four overs.
Surrey's other star batsman Kumar Sangakkara then fell cheaply,
leaving Pietersen to take centre stage as the rest of the batting line-up
struggled.
However, Essex's batsmen found it no easier after Dan Lawrence
(24) and Varun Chopra (28) got them off to a good start. Gareth Batty, Tom
Curran and Stuart Meaker all took two wickets to restrict the visitors.