Essex 211 for 5 (Pepper 120, Allison 44) triumphed over Sussex 207 for 2 (Hughes 81, Clark 72) by five wickets.
Essex, needing to win their final three games for a home quarter-final spot in the Vitality Blast, started strong with an unexpected victory over the Sussex Sharks by five wickets with nine balls to spare.
Michael Pepper led Essex with an extraordinary unbeaten 120 off just 53 balls, hitting nine sixes and nine fours. He received crucial support from Charlie Allison, who contributed an unbeaten 44 off 26 balls, including a six and four fours. Their partnership of 121 runs in nine overs astonished the sell-out crowd.
Sussex had set an impressive 207 for 2. When Essex began their chase, they faced early setbacks. Ollie Robinson struck with the second ball, catching Adam Rossington at deep square leg. Tymal Mills followed by bowling Dean Elgar for 11 in the fourth over, reducing Essex to 30 for 2.
Robinson, switching to the Cromwell Road End, took two more wickets with his first two deliveries, dismissing Robin Das and Paul Walter. At 90 for 5 in the tenth over, Essex seemed in trouble. However, Pepper, who had aggressively attacked spinner James Coles, continued his impressive form, reaching a strike rate of 200 for the season and accumulating 488 runs at an average of 48, including his second century.
Sussex also needed two wins from three games for a home quarter-final spot. Their innings were built around a dominant partnership for the second time in two days. On Friday, Tom Alsop and John Simpson put on 139 against Kent. Against Essex, Daniel Hughes and Tom Clark added 110 for the second wicket in just 11 overs.
Sussex had a rapid start, with Hughes and Harrison Ward scoring 34 in 3.2 overs before Ward was caught at mid-off. Hughes powered Sussex to another big total with an 81 off 43 balls, featuring five sixes and eight fours, maintaining a strike rate of 188 and bringing his season total to 435 runs.
Hughes hit Shane Snater for two sixes in one over, bringing up his fifty in 26 balls. Sussex reached 94 for 1 at the halfway mark, with Hughes hitting leg-spinner Matt Critchley for a massive six to reach the hundred.
Clark, on a slightly slow pitch, initially struggled with his timing. However, after hitting Walter for six over fine leg, he accelerated to an unbeaten 72 off 53 balls, with eight fours and two sixes. Alsop added a quick 27 not out off 14 balls, including a six over midwicket off Snater, but it wasn't enough to secure victory for Sussex.