Stephenson, under fire following last week's exit from the NatWest Trophy at the hands of Glamorgan and in a personal slump of form with the bat, claimed 6 for 33 in a 7.3 over spell.
The former England batsman brought to an end a potentially match-winning innings from Graeme Hick, who smashed a six and three fours on his way to 48. He and Tom Moody put on 92 for the second wicket but Worcestershire struggled thereafter and were all out for 145 as they chased Hampshire's target of 232 for 4.
Despite the efforts of Bobby Chapman (3 for 27), the home side's total arrived with Matthew Keech (53) and Shaun Udal (52) putting on 98 for the fourth wicket. Robin Smith made 49.
A half-century from David Hemp carried Warwickshire to the top of the Sunday League as they achieved a target of 137 in 17 overs with three balls to spare against Durham at Chester-le-Street.
The five-wicket win in a rain-interrupted match looked extremely unlikely when Hemp went in at 37 for 2 in the eighth over, but the former Glamorgan left-hander hit the first two balls he faced from James Boiling for two and six, and reached his first one-day fifty for his new county off only 20 balls.
The match was initially reduced to 32 overs a side after an hour's delay and another downpour terminated Durham's innings when they had made 174 for 2 from 27.4 overs. Left-hander Stewart Hutton hit eight fours in his 57 and Martin Speight made 64 not out off 56 balls.
Yorkshire's hopes of claiming the title took a heavy blow when they fell to defeat by 114 runs against Derbyshire. David Byas's team went into the match six points behind this morning's leaders Essex, who were not in action, but they were never in a one-sided contest and lost with 7.2 overs to spare.
Derbyshire dominated from the moment Byas put them in with Adrian Rollins and Chris Adams racing to 50 in only seven overs before Richard Stemp and Peter Hartley applied the brake. Hartley had Rollins taken at short mid-wicket for 36 and Gul Khan fell to a low catch by Craig White at short extra cover for six but that was the end of Yorkshire's success.
The rest of the attack was savaged by Adams and Vince Clarke, who added 100 in 13 overs. Darren Gough conceded 65 runs from eight overs as Adams and Clarke shared an unbroken stand of 158 - a Derbyshire third-wicket record in the Sunday League. Adams' century, which followed a NatWest Trophy hundred on Wednesday, came off 104 balls while Clarke's 77 occupied only 68 deliveries.
Derbyshire's 259 for 2 was also their highest Sunday League total against Yorkshire, who replied disastrously, losing Byas and White in the first four overs.
When Philip DeFreitas had Richard Blakey caught behind for nine and Bradley Parker played on three balls later, Yorkshire were 39 for 4. Lehmann (30) and Anthony McGrath (40) prevented complete humiliation but Andrew Harris ripped through the lower order to finish with 4 for 22 as Yorkshire subsided in a flurry of flying stumps to 145 all out.Reuse content