Southampton threw away an early lead to lose four-one at home to Tottenham Hotspur this evening, with Nathan Redmond sent off before the hour mark.
Goals from Kane, who also missed a penalty, Son and a Dele Alli brace gave Spurs a deserved win with Southampton unable to maintain their excellent start to the game.
Saints took an early lead through Virgil van Dijk, who rose above Jan Vertonghen to power home just two minutes in. Ryan Bertrand was tripped, with James Ward-Prowse's in-swinger finding the Dutchman who netted his first of the season.
The visitors were shell shocked, with Southampton harrying and harassing Spurs impressively, and much the better team until a somewhat fortunate Tottenham equaliser arrived in the 18th minute.
Moussa Sissoko found space to put in a cross from the left, with Nathan Redmond slow to close him down. The ball took a heavy deflection which appeared to catch van Dijk cold but Alli on his toes to head in off of the bar.
From then on in, Southampton sat deep and looked to counter, and they joined Spurs in looking uncomfortable with the ball, misplacing passes and inviting pressure. Fraser Forster was again nervous kicking, with Oriol Romeu and others saving his blushes.
Redmond narrowly missed with a decent effort from distance half an hour in, while former Saint Victor Wanyama skinned van Dijk only for Jose Fonte to shuffle across to deny the Kenyan merely yards from goal just before half time.
The second half started terribly for the home side, with Kyle Walker nearly capitalising on Bertrand's trepidation in dealing with a long ball in over the top. Minutes later, disaster struck.
Harry Kane met a Christian Eriksen corner at the near post, rising above Romeu in the process and powering the ball home. Jose Fonte had a chance to level seconds later from a Ward-Prowse free kick, but his own effort was wild.
Things got worse still in the 54th minute, when Sissoko led a breakaway and fed Alli who ran through into the Saints' box. Redmond tugged Alli's arm as he approached, let go, and then stumbled into the Englishman, meaning that the foul had continued into the box. Mike Dean was right to award a penalty, but Redmond was a touch unlucky to be sent off too since he took a tumble and wasn't clearly the last man.
Kane missed the penalty, blaming the groundsman and an apparently dodgy penalty spot, and fans wondered if the miss would prove decisive. It didn't.
From there on in, it was all Tottenham. The visitors slowed things down and picked their moments to strike and Southampton were easily overpowered. Danny Rose's cross landed at Eriksen's feet, with his shot crashing into the bar and Saints were switching off. Wanyama and Eriksen both danced through challenges and Spurs players easily escaped their markers on several occasions.
Substitute Heung-Min Son added a third, sliding in beyond Forster's far post after a breakaway that saw Oriol head a diagonal pass directly into the striker's path. Unluckier still was how Pierre-Emile Hojbjerg failed to equalise seconds earlier - Shane Long harried Hugo Lloris into a poor headed clearance, but the Dane totally fluffed his chance and Saints were punished.
Saints huffed and puffed, but could not break down an organised and combative Tottenham team. The final arrived from the boot of Dele Alli, who, alongside Wanyama bossed the second half for the visitors.
In the end, Spurs were worthy winners but not without their slices of luck. Southampton conceded four in the league at St Mary's for the first time since returning to the Premier League after being tight at the back this season.
They will have to sharpen up quickly with West Brom visiting on New Year's Eve, quickly followed by Ronald Koeman's Everton just two days after that.
Southampton: Forster, Cedric, Fonte (C), Van Dijk, Bertrand, Davis (Hojbjerg), Romeu, Ward Prowse, Boufal (Tadic), Redmond (Red), Rodriguez (Long).
Subs: Hojbjerg, Long, Martina, Reed, Tadic, Taylor, Yoshida.
Spurs: Lloris (C), Walker, Dier, Vertonghen, Rose, Wanyama, Dembele (Winks), Sissoko Son), Eriksen, Dele, Kane (Janssen).
Subs: Davies, Janssen, N'Koudou, Son-Heung-Min, Vorm, Wimmer, Winks.