The side were boosted by the inclusion of South African all-rounder Imraan Manack, whose arrival as the club's overseas player had been held up by administrative delays in his homeland.
However, it was a familiar tale once Horspath had decided to bat as they collapsed to 149 all out against impressive Harefield seamers Tom Marshall (4-46) and Jordan McLeod (4-48).
Openers Jimmy Phillips and Shiryar Ahmed put on 30 for the first wicket, but after the latter edged a McLeod delivery into Karanpreet Singh's gloves the rot set in, and the home side were quickly 59-6.
Phillips looked as if he was set to carry on the good form he had shown in the midweek T20 win as he moved on to 26 only to pick out Aatish Anthathi at mid-off to Marshall's delight.
The wickets continued to tumble, and Manack was unable to provide an instant answer as he edged behind off McLeod for four.
It was only when Max Smith and skipper Patrick Foster came together that the home side started to show some resistance, guiding Horspath to lunch at 90-6.
They had added 56 for the seventh wicket, before Smith's gritty knock ended when he became the third player to fall to the McLeod-Singh combination for 22 with the wicketkeeper diving low to his right to take a brilliant one-handed catch and make it 115-7.
Foster had batted fluently to make 28, but he followed four runs later when he was bowled by his Harefield counterpart, Stephen Croft (2-25).
Amin Rafiq showed good application to make an unbeaten 17, before Horspath were finally all out with 10.4 overs left. To add to their woes their final total was one short of a second batting point.
Horspath needed an early breakthrough to stand any chance and they found it when Justin Clarke rushed one through Anthathi's defences during an excellent opening spell to make it 13-1.
But their hopes proved short-lived as Matthew Ogden and Thomas McLeod shared a second-wicket stand of 104. Foster rang the changes in an attempt to make the breakthrough, with Manack bowling seven tidy overs on his debut without reward.
However, it wasn't until fellow off-spinner Rafiq came into the attack that the home side enjoyed some success with McLeod stumped by Rob Wight for 40.
Rafiq struck again to remove Ogden for 64 from 106 balls with Ahmed taking the catch to leave Harefield 128-3. His patient knock included 12 fours.
Left-arm spinner Lloyd Belcher then took two wickets in two balls, including a second stumping for Wight, and Rafiq claimed a third victim when Nick Kelly was lbw for 23 before Harefield secured victory at 150-6. Rafiq finished with 3-31, Belcher 2-29 and Clarke 1-24.
Despite the defeat, Horspath remain in sixth place ahead of next week's trip to seventh-placed Great Brickhill.
Posted by: Russ