Surrey 161 for 4 (Roy 72, Evans 73) beat Kent 159 for 8 (Billings 50, Plunkett 3-19) by six wickets
Jason Roy's first half-century of the summer helped Surrey to confirm top spot in the South Group and lock in a home quarter-final in the Vitality Blast.
Chasing 160 to win - and 127 to ensure they won the group ahead of Kent on net run-rate - Surrey started slowly, losing the in-form Will Jacks early on and with Roy looking out of nick as he struggled towards 18 off 21 balls.
But Roy started to find his rhythm with a flurry of boundaries off Fred Klaassen in the fifth over, and was soon up and running. He was ably supported by Laurie Evans, who has now made 268 runs for twice out in his last four T20 innings after a lean start to the Blast, with their 135-run partnership setting up a convincing victory.
It has been a difficult summer for Roy, who managed only 49 runs in his six ODI innings for England, and missed all of their T20Is due to a side strain. His two previous Surrey appearances had yielded 24 runs in three innings, across the Bob Willis Trophy and the Blast, and he would be the first to admit that things had not gone his way.
"He's a bit grumpy at the minute, and we've lost four golf games in a row, so he's not speaking to me on the golf course either," Evans joked. "Me and Jase said it was a wicket that if you bowled well on, it made it quite difficult to score. We took options at the right time and put them under pressure, and I'm glad we got over the line."
Roy's best shot came off Imran Qayyum, when he skipped down the pitch to loft the left-arm spinner into the building site to the right of the pavilion, and he reached his half-century with a slash through extra cover off the same bowler.
Evans, meanwhile, appears to have found his groove after a quiet start to the competition. He batted superbly for his 45-ball 73, with a towering six over extra cover off Matt Milnes to bring up his half-century demonstrating his form: he picked a 61mph/98kph slower ball outside off, hung back in his crease, and launched it into the stands with a full extension of his arms. He smote another into the top tier of the pavilion, putting his inexplicable drop of Sam Billings in the first innings to the back of his mind.
Kent's 159 had appeared a par score on a slow, used pitch, and owed plenty to Billings' 33-ball 50. After Daniel Bell-Drummond continued his impressive season with a fluent 37 at the top of the order, Kent had begun to stumble in the middle overs with two wickets in as many overs.
Billings drove them up towards a useful total as he extended his form with the bat, taking a particular liking to Jamie Overton, Jacks and Reece Topley, all of whom were hit for two fours in an over, but they ultimately paid for a poor finish.
From 136 for 3 after 16 overs, they failed to hit a boundary off the bat in the final four overs, with Liam Plunkett in particular impressing with a series of cutters and back-of-a-length balls. Kent paid the price, too, for picking an extra batsman: Heino Kuhn and Jack Leaning contributed two runs from four balls between them, while captain Billings had no choice but to give Joe Denly his full four-over allocation.
"Joe Denly has been brilliant with the ball so we get four overs out of him, and Jack Leaning's also bowled well with his offspin but with two right-handers and the way they were playing, I thought it was a risk to get a couple of overs out of him [today]," Billings said, defending their combination.
"Timmy Groenewald is on the bench at the moment, and maybe that's something we can look at - that experience, coming into those latter stages might be something to lean on - but I think the boys have done really well in this tournament, and trying to get back to what they were doing, that's all."
The fixture also served as a warm-up act for next month's quarter-final, with the same two teams returning to The Oval on October 1.
Vitality Blast quarter-finals:
Nottinghamshire v Leicestershire
Surrey v Kent
Gloucestershire v Northamptonshire
Sussex v Lancashire
All fixtures to be played on October 1
Matt Roller is an assistant editor at ESPNcricinfo. He tweets at @mroller98