Essex 315 for 6 (Pettini 88, Westley 62) beat Northamptonshire 311 (Willey 113, Westley 4-60) by four wickets
Essex chased down 312 to set up a home tie in the quarter-finals of the Royal London Cup after a four-wicket victory against Northamptonshire at Wantage Road in their last Group A outing.
Victory made sure Essex will play at Chelmsford in the knockout stage, and Warwickshire will be the visitors in a repeat of the NatWest T20 Blast quarter-final.
Northamptonshire also had hopes of going through at the start of the day but Derbyshire's defeat of Gloucestershire put paid to that hope
Having been put in, the hosts racked up a 311 from 47 of their 50 overs. David Willey was the star of the show with a 93-ball 113 - his second List A century - dominating the innings as an indifferent Essex performance in the field was exploited.
After Richard Levi and Rob Keogh put on 100 for the first wicket in less than 15 overs, the latter was the first to go for 50. Kyle Coetzer and Adam Rossington then fell cheaply, before Willey took centre stage with a perfectly paced effort.
He had by far the better of a 114-run partnership with Levi, the South African contributing a meagre 27, and after reaching 50 from 44 deliveries, he needed 38 more to make his way to three figures.
Levi, who did not hit a boundary for 60 balls after an initial burst, made 84 at nearly a run a ball but after he and Willey had departed, the last five wickets aimlessly fell for the addition of just 13 runs - with Tom Westley, who finished with 4 for 60, picking up the final four in a 13-ball spell.
Essex's reply started brightly and with the in-form Westley - who romped past the 50 mark - leading the way, 94 had been wiped from the total required when Matt Spriegel got rid of the opener in his first over. This was the cue for Pettini to move up a gear and after surviving a life on 35 as Rossington put down a sharp chance at cover off Graeme White, he also made his way past 50 with relative ease.
In conjunction with Ravi Bopara, the required rate was kept to with little difficulty and although Pettini, caught from a leading edge off White for 88, and Jaik Mickleburgh - yorked for seven by Willey - departed, the visitors were still in control.
Bopara strolled to half-century number three but he fell shortly after Kishen Velani to tighten everything up. Although James Foster fell attempting something ambitious, the winning line was crossed with three balls to spare.