Middlesex Panthers lost both games of their NatWest T20 Blast double header at Lord's on Saturday.
Essex Eagles chased down a target of 181 for an eight-wicket victory and Sussex Sharks prevailed by 13 runs in a contrasting affair later in the afternoon.
Middlesex's day actually started well as Dawid Malan (86) and Eoin Morgan (77) put on a second-wicket stand of 118 to set up a total of 180-5 after they had been sent in by Essex skipper Pettini.
It would have been an even bigger score but for left-arm seamer Reece Topley (3-26) hitting back with three late wickets.
Pettini, leading the Eagles until Ryan ten Doeschate's IPL stint concludes, launched his side's run chase with an 81-run stand at 10-on-over alongside opening partner Alastair Cook.
Cook (22) and Essex debutant Jesse Ryder (15) both fell to left-arm spinner Ravi Patel (2-26) but Pettini and Ravi Bopara (45no) combined in a 69-run stand to complete the win with seven balls to spare.
Pettini finished 95 not out from 54 deliveries, including 14 fours and a six.
Middlesex faced a quick turnaround of just over an hour before they took on Sussex Sharks and again started the game strongly, a much improved display from their seamers limiting Sussex to 128 all out in 19.4 overs.
Toby Roland-Jones took the final two wickets with successive balls to earn figures of 3-26, while James Harris (2-10) and Dan Christian (2-23) each picked up two wickets.
But Middlesex's batsmen - so impressive earlier - floundered on the way to finishing short at 115-6, unable to come to terms with a disciplined bowling display from Sussex.
Medium pacer Steffan Piolet claimed the key wicket of Middlesex captain Morgan (30) via a top edge in the 16th over among his 2-15 haul.
In the North Group, Lancashire edged to a 12-run victory over gritty Worcestershire at Old Trafford.
Paul Horton and Tom Smith's 128-run stand put the hosts in control at 194 for three from 20 overs.
Opener Smith made 74 before Saeed Ajmal had him stumped by Ben Cox, while Horton fired an unbeaten 71,
Worcestershire refused to roll over though, with Richard Oliver making 34 and Colin Munro 39 in a solid card.
Alexei Kervezee posted 32 not out and Ross Whiteley an unbeaten 17 as the visitors battled manfully to drive over the line at the death.
Lancashire were just able to hold on however, to register their first win in this year's tournament.