The South African all-rounder, who celebrated his 30th birthday yesterday, rescued the Rapids after they had slipped to 35-3 in reply to Derbyshire’s 156-4.
He reached his half century off 34 balls with the aid of four sixes and one four
Ben Cox and Ross Whiteley proved crucial support but then the Rapids lost four wickets for seven runs when the finishing line was in sight before victory was secured with one over to spare.
Parnell finished unbeaten on 81 from 46 balls with seven sixes and two fours to move the Rapids within one point of leaders Lancashire Lightning.
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The Falcons were put into bat on the same pitch used for the Rapids run blitz against Durham on Sunday.
Dillon Pennington took the new ball and his third legitimate delivery accounted for captain Billy Godleman (0) who shaped to cut and was caught behind.
Luis Reece found the England Under-19 paceman to his liking in his second over as he collected two fours and a huge six over mid-wicket.
But he perished to the last ball of the power play on 29 when deceived by a slower ball from Pat Brown which he scooped straight to mid-off.
Skipper Brett D’Oliveira came into the attack and Wayne Madsen (28) fell to a fine diving catch by Parnell at cover with the South African celebrating with a series of press-ups!
The leg spinner struck again when Matt Critchley (9) tried to repeat his straight six from earlier in the over and was pouched at long off.
The ball was sticking in the pitch and batsmen were struggling to time the ball.
Veteran opening batsman Daryl Mitchell tied down the Derbyshire batsmen with a mid-innings spell of four overs for just 15 runs.
But Leus du Plooy and on loan Kent all-rounder Darren Stevens gave the innings some late momentum with an unbroken stand of 70 in seven overs.
Du Plooy brought up a 46 balls half century with five fours and Stevens ended unbeaten on 26 from 18 deliveries.
D’Oliveira ended with 2-25 from three overs, Brown 1-28 from four and Pennington 1-20 off two.
Guptill had smashed an unbeaten 86 off 31 balls against Durham and started in the same vein with a straight six off Ravi Rampaul.
But the introduction into the attack off Fynn Hudson-Prentice saw him claim the scalps of Guptill and Riki Wessels in the space of four balls in his first over.
Guptill (15) holed out to long on and Wessels (4) miscued to cover with the total 20-2 in the third over.
It became 35-3 when Callum Ferguson (6) was caught off a leading edge at cover from the first delivery by Reece.
Parnell added some impetus to the Rapids innings and a six over mid-wicket off Reece brought up the 50 in the eighth over.
Cox gave him good support as the half century between the pair came up in six overs.
The wicket-keeper was caught around the corner off Mark Watt (20) but in the same over Parnell completed a 34 ball fifty and then hit the same bowler for his fifth six to bring the 100 up in the 13thover.
Whiteley proved to be a useful ally to Parnell in a stand of 49 in just four overs and made 20 off 12 balls before he was bowled by Watt and then Ed Barnard (0) was run out from the following delivery.
Mitchell (5) fell to Rampaul with 10 still needed and then D’Oliveira (1) was bowled by the West Indian paceman at 149-8.
But Parnell drilled Alex Hughes for six over long off amidst great excitement and relief amongst the 3,000 crowd to settle the contest in the Rapids favour.