Koen made 44 from just 17 deliveries to seal victory over Keelby with just two balls to spare.
The result left them fourth in Lincs County League Division One, 37 points behind second-placed Cleethorpes.
Winning the toss, skipper Arran Brindle put Keelby in to bat and the afternoon started well as the former England international struck in her first over, having Travis trapped lbw.
After a sustained spell of pressure bowling, containing five maiden overs, Brindle struck twice more to leave Keelby rocking at 33-3.
Andy Bagnall then took control for the visitors, using his favoured cut shot to make a well-earned 58 as the Louth fielders toiled.
The Wolves started to gain a foothold as Keelby slipped to 132-6 with wickets for slow bowlers Siddiqui and Fisher.
However, visiting skipper Bevis had other ideas and began to take on the home bowlers, scoring a brisk 71 before departing to a steepling high catch to Bains off the bowling of Koen.
This didn’t stop the flow of runs as lacklustre fielding from the home side and a rearguard from Eames (43 not out) and Greenhaigh (13 not out) helped the visitors post a challenging final total of 212-8.
The Louth reply did not start well as Bains departed before the score had reached double figures.
However, regular opener Wright steadied the ship with newcomer Cearnes and the pair moved the score to 49 before Cearnes departed.
This brought Medler to the crease, and with Wright, the pair chipped away at the imposing total and by drinks in the 21st over had reached 76-2.
The pair pushed on after the break and the hosts looked on the up when Wright departed for a well-earned 68 with the score on 115.
In-form batsman Siddiqui failed to trouble the scorers and with overs running out, Keelby now looked favourites for the win.
Needing more than six runs per over, Medler and new batsman Fisher upped the ante, running well and keeping up with the required rate.
With Koen waiting in the wings, Fisher had no choice but to take on the visiting attack and departed for a brisk 17.
At 158-5 the game looked set for a thrilling climax with seven overs left. Medler and Koen continued to keep up with the rate, particularly Koen who punished the bowling with powerful boundaries.
The tension rose as Medler departed for a hard-fought 68 and skipper Brindle followed soon after with the score on 199-7 as the game entered the final over.
Requiring 13 runs to win, Koen moved into top gear, smashing a six and two fours, to earn a standing ovation and a dramatic win.
On Saturday, the Seconds travel for the return leg.