Of all the matches that were supposed to bring this Champions Trophy to life, Sri Lanka versus India was not it. Pakistan's upset of South Africa had spiced up Group B - though the Birmingham rain kiboshed the chances of a genuine thriller - but it seemed going into this fixture at The Oval that the chances of Sri Lanka overturning their great bogey opponent, one of the favourites for the tournament, were only marginally higher than Jeremy Corbyn's Labour party storming to a landslide victory in the UK general election.

These are tough times for the discipline of psephology - and who knows what British exit polls may be about to tell us - but at least the underdog victory remains one of sport's great pleasures. Sri Lanka came into the Champions Trophy with the worst record of the eight participants since the last World Cup and walked into this match having lost 16 out of 21 ODIs against India, going back to their defeat in the 2011 World Cup final.

But while Sri Lanka's bowling effort was largely forgettable, as India ambled toward what many presumed to be an adequate total, they suddenly came to life in the chase, led by two young batting bucks in Danushka Gunathilaka - who was only added to the squad as an injury replacement on Wednesday - and Kusal Mendis. Both were run out short of a century, both short of the crease despite desperate dives, but India had been rattled and belief began to course through Sri Lankan veins.

Their supporters, too, sensed the moment. Although the game began with the pounding rhythm of Indian dhols dominating the atmosphere, a little over an hour later a papare band finally struck up a tune on the north side of the OCS Stand. They were still going when Angelo Mathews nonchalantly tucked a single to complete victory and by now, with clumps of departing India supporters beginning to leave gaps in the stands, Sri Lanka flags were popping up all over, waving proudly in the evening breeze. Sri Lanka may have been traditionally late to the party - but the party has now officially started.

Not only did they beat India, setting up the tantalising prospect of two virtual quarter-finals, but they did it equalling their highest successful chase and with eight balls to spare. Notably, the cool contribution of the returning Mathews aside, this was a landmark moment for the new generation of Sri Lanka batsmen, after a couple of difficult years that have encompassed the retirements of Kumar Sangakkara, Mahela Jayawardene and Tillakaratne Dilshan.

There is no doubt that losing three such big beasts is a significant handicap, no shame in admitting it either. When Sri Lanka reached the semi-finals of this competition four years ago (where they were rolled over by India) that august trio were their leading scorers. As they have begun to rebuild since, Sri Lanka have carried around a convenient excuse for failure, a note to be handed to the teacher before the start of class: "Sorry, Miss, we didn't bring Sanga and Mahela."

It must be difficult, too, seeing Sangakkara touring county cricket and scoring hundreds with regal disdain - only 10 days ago, he missed out on a record-equalling sixth successive first-class ton by a mere 16 runs. Before this match, on what is now Sangakkara's home ground with Surrey, Mathews was asked whether they had tried to get the old maestro in as a ringer. "I always asked him that," Mathews replied. "Even a couple days ago, I was asking him can you come and play for us, you know, in this game?"

Mathews was joking, but many a true word was said in jest. Now, perhaps, Sri Lanka will have the conviction they can do without. When Jayawardene turns out for Lancashire in the NatWest T20 Blast later this summer (as a local no less) Sri Lanka fans may be able to chuckle rather than be reduced to tears.

Afterwards, Mathews had special praise for Gunathilaka. "To walk into a game knowing he wasn't even in the 15 - Kapugedera got injured yesterday and he had to step in and open the batting against a very strong opposition - it was fantastic the way he batted, it was very pleasing. Kusal Mendis and Danushka played with a lot of freedom and that really set us a platform, made it so much easier for myself and Kusal Perera and the middle order. Don't forget Asela [Gunaratne]. That little cameo really won us the game. It was a fantastic batting effort."

Having been comfortably beaten here by South Africa at the weekend, Sri Lanka stuttered out of the blocks again. India were inserted after Mathews won the toss but much of what followed felt pre-programmed. Their fast bowlers bent the knee, rather than their backs - though on an admittedly docile track - and never looked like making the inroads that overcast conditions might have promised. Lasith Malinga's first ball was a wide long-hop that Rohit Sharma eased to the boundary.

Malinga began his second spell with a wide, too, then commenced his next over with a juicy half-volley that Shikhar Dhawan spanked through the covers to bring up his fifty. Malinga's resurrection for this tournament, having not played an ODI for 18 months, feels increasingly like rolling out a battered T-800 with an Arnold Schwarzenegger mask strapped on, the returns diminishing along with later instalments of the Terminator franchise.

But if the aging stars are no longer guaranteed box office, then it's time for a reboot with new men in leading roles. Gunathilaka and Mendis made eye-catching contributions on this ground a year ago, though their pair of half-centuries were ultimately dwarfed by Jason Roy's 162. Gunathilika's fast hands set the tone for Sri Lanka's chase this time around, flashing the first boundary in the fifth over, a riposte to Bhuvneshwar Kumar's dismissal of Niroshan Dickwella.

Predictions about India's much-vaunted attack were confounded, the bowling made to look flatter than your average election pollster, wearily trying to come up with a formula that works in the modern age. Gunathilaka stepped out to straight drive Jasprit Bumrah, using 144 kph to his own advantage, and then slapped Hardik Pandya for six to go to fifty. Mendis had a life on 24 when Pandya could not hold a return chance but then set about neutralising Jadeja, three fours and a six whipped impudently through the leg side to expose a previously hidden imbalance in India's XI.

Throughout the second half of the match, as the unthinkable became imaginable, then inevitable, the lone troupe of papare players kept up their jaunty rhythm, an island of resistance in a sea of India tricolours and bhangra drumbeats. "Nava gilunath baan choon," they say - even if the ship sinks, the party will go on. Well, Sri Lanka are still afloat and the party is in full swing.