World Cup warm-up match, Bloemfontein: Sri Lanka 183-5 (38.2 overs) beat Free State 182-7 (50 overs) by five wickets
Arnold took two wickets and then led Sri Lanka to victory
Sri Lanka recovered from 77 for four before ultimately coasting to victory against Free State.
The Sri Lankans had been set an easily attainable 183 for victory after Pulasthi Gunaratne and Russel Arnold led their bowling attack with two wickets apiece.
Free State's Johannes van der Wath had the opposition wobbling when he grabbed the wickets of opener Sanath Jayasuriya and Hashan Tillakaratne in quick succession.
But Arnold brought his bowling form into his batting, steering his side to the win with Mahela Jayawardene on an unbeaten 58.
Morne van Wyk and Jonathan Beukes looked on course to set a big total for Free State when they posted 92 for the first wicket.
However, Van Wyk, having scored 46 including five fours and a six, became Arnold's first victim, caught by Dihan Gunawardene and with the fall of the opening partnership Free State's innings collapsed.
Johannes van der Wath was the next back in the pavilion in the following over after being Muttiah Muralitharan trapped him lbw.
Beukes and Tshabalala followed shortly afterwards to leave Free State languishing on 115 for four.
Gerhardus Liebenberg and James Schorn added a painfully slow 38 runs in the next 15 overs to stop the fall of wickets.
After Schorn became Gunaratne's first victim in the 44th over with the scores at 153, Free State lifted their tempo to leave the Sri Lankans chasing 183 for the win.
Free State: MN van Wyk, JA Beukes, JJ van der Wath, ST Tshabalala, GFJ Liebenberg, JR Schorn, R McLaren, H Bakkes , D Pretorius, A Abrahams, PV Mpitsang.
Sri Lanka: ST Jayasuriya, DA Gunawardene, HP Tillakaratne, PA de Silva, DPMD Jayawardene, KC Sangakkara, TCB Fernando, PW Gunaratne, RP Arnold, M Muralitharan, RAP Nissanka