The 3-1 result in no way reflected the performance on a ground we have yet to win at either reserve or first team level. That unwanted record looked all set to be put to rights when Thatcher thumped home after nine minutes of a first half dominated by Colin Calderwood's side.
The go-ahead for the game was subject to a pitch inspection 30 minutes before kick-off and the referee Mr Tomlinson declared the surface playable despite one penalty area being submerged in water.
Jamie Slabber headed our first chance of the match just wide of the post from an Etherington cross as Spurs began brightly in the pouring rain.
Much of the local interest centred on Southampton's new Argentinean recruit Federico Arias, but he cut an isolated figure in the opening stages as Spurs bossed play.
Lee Barnard, who formed a youthful striking partnership with Slabber, was manhandled by Williamson on the edge of the area and the free-kick offered Thatcher a clear shooting chance which he did not decline.
From a short run up, the left back thumped a low drive past the fruitless dive of home goalkeeper Paul Jones. The lead was nearly surrendered straight away, but Sullivan was particularly alert to foil Delgado.
Brilliant interchange between Blondel, Sherwood and Ricketts created a scoring chance for Slabber on 15 minutes, but the Youth team striker's curling effort drifted just wide with Jones clutching thin air.
Sullivan was again called into action on 24 minutes when he had to get down sharply to deny Davies - just as the downpour got considerably heavier. The increase in rainfall appeared to spark a Spurs purple patch of possession and a delightful exhibition of one-touch football followed, with Blondel and Etherington central to most of the easy-on-the-eye display.
Southampton began the second period in a livelier manner and, within two minutes, parity was restored. Davies dinked the ball in behind Doherty into the patch of Delgado, who lofted his effort over Sullivan to level.
Composure was regained shortly after, with Barnard and Blondel again pulling the strings - the striker often dropping deep to provide the link play. A Blondel shot from distance was well fielded by Jones. Arias then went close with a twice-taken free-kick from the edge of the area following a Blondel foul on Davies.
Barnard had the ball in the net on 65 minutes when he hunted down Jones while in possession outside his area on the byeline. Lee nicked it off him, but the referee adjudged the ball to have gone out of play. Slabber then fired straight at the Welshman under pressure when put through by Ricketts.
Delgado, who was posing more and more of a threat, acrobatically fired over the bar as the game entered the final 15 minutes.
On 78 minutes he closed down Sullivan in the process of making a clearance and the ball squirted loose. The striker got there first with Sully in hot pursuit but, instead of turning and shooting, Delgado laid the ball back to Howard who put the ball back into the danger area where Davies was on hand to trickle the ball past the despairing keeper.
Southampton gave the scoreline a deceptive look when a Howard free-kick three minutes from time was met by the head of Baird, who flicked past Sullivan.
Spurs: Sullivan, Thelwell, Thatcher, Ricketts, Doherty, Gardner, Blondel, Sherwood (Jackson 71), Slabber (Yeates 78), Barnard, Etherington. Subs: Burch, Henry, Hughes.
Southampton: Jones, Dodd (A Davies 67), Benali, Byles, Williams, Baird, Arias (Anderson 67), Crowell, Delgado, K Davies, Howard. Subs: Mills, Blayney, Gleeson.
A superb display of free-flowing football and a goal from Ben Thatcher, but the reserves were undone by three second half strikes by Southampton at St Mary's on Tuesday evening.