NORTHAMPTON - Rob Keogh's century ensured Sri Lanka encountered determined Northamptonshire resistance as their final match before next week's first Test against England at Lord's ended in a draw. That was always the most likely outcome after Saturday's third day of four was washed out without a ball bowled and became all but inevitable after Sri Lanka opted against enforcing the follow-on on Sunday. Sri Lanka made 558 for eight declared with Lahiru Thirimanne -- captain for this match in the absence of the rested Angelo Mathews -- top-scoring with a career-best 156 after being dropped first ball and opener Kaushal Silva 152 before retiring hurt. Northamptonshire responded with 345 all out featuring Rob Keogh's 120 -- the first first-class hundred by a Northamptonshire batsman this season. Paceman Dhammika Prasad (four for 68) and off-spinner Dilruwan Perera (four for 92), both battling for a Test place, were Sri Lanka's most successful bowlers. Sri Lanka were 99 for four in their second innings, with Prasanna Jayawardene four not out and Perera nought not out, when a draw was agreed shortly after 5:00pm local time (1600GMT) with 17 overs left in the day's play.Dinesh Chandinal was promoted to open in Sri Lanka's second innings. But he'd made just eight when he chipped left-arm seamer David Willey, returning to bowling this match after a back injury, to short mid-wicket. After tea, opener Dimuth Karunaratne (38) played on to left-arm spinner Matthew Spriegel to end a second-wicket stand worth 75.Then 89 for two soon became 90 for three when Mahela Jayawardene charged down the pitch to left-arm spinner Graeme White and was stumped for nought by wicketkeeper Ben Duckett. White also dismissed Kumar Sangakkara, Sri Lanka's other senior batsman, when the left-hander holed out to long-on for 45 shortly before the close. Northamptonshire resumed Sunday on 146 for five, with Keogh 26 not out and Matthew Spriegel unbeaten on 11.Sri Lanka might have hoped to run through Northamptonshire, struggling at the foot of the First Division of the County Championship with five defeats from six matches after a series of batting collapses. However, Northamptonshire's sixth-wicket duo defied Sri Lanka's seamers, with Keogh cover-driving left-arm paceman Chanaka Welegedara for four. But on a sunlit day, Perera succeeded where the quicker bowlers had failed when he somehow induced Spriegel (27) to chip tamely to Nuwan Pradeep at mid-on and so end a stand of 86.Soon afterwards, Northamptonshire lost their seventh wicket when Steven Crook edged a drive off Prasad and was well caught by diving wicketkeeper Prasanna Jayawardene. But the aggressive Willey -- the son of former Northamptonshire and England batsman Peter Willey -- drove Perera for a huge straight six. Willey had made 48, including three fours and two sixes, when he recklessly charged at Perera only to be stumped by Prasanna Jayawardene.Keogh, playing only his second first-class match of the season following a finger injury, was still 20 runs shy of his century when last man Chad Barrett came in. But Barrett, on his first-class debut, kept the 22-year-old Keogh company with several shots that belied his status as a No 11. Keogh went to his second first-class hundred, following his 221 against Hampshire last year, courtesy of three offside fours in an over off seamer Pradeep.His more than five-hour innings, as well as a last-wicket stand of 64, eventually ended when he was caught behind flirting outside off stump off Prasad. Keogh batted for just over five hours and faced 202 balls including 18 fours. Barrett was 20 not out.