A late Southampton charge was not enough to deny Trojans their deserved passage into the last four of the Hampshire Worthington's Bowl as the Stoneham Lane side ran out 18-17 winners.
In the proverbial 'game-of-two-halves', the Hampshire Division One leaders tore into their higher league opponents with two tries in the first half through winger Ian Hudson and number eight Ollie Joisce and the visitors had little in response.
It was a different matter after the break after Nigel LeBas had extended the lead to 18-7 with a drop goal as the Saints pinned Trojans deep into their half and winger Julian Robins capitalised with a brace to bring the game to within a point.
But Trojans' defence held out as Tony Cador's men tried hard in the final quarter but it was not to be and moved through to a mouthwatering semi-final meeting with fellow league promotion hopefuls Guernsey at the end of February - just two weeks after they meet in Hampshire One.
"We are very pleased to have won," said Trojans coach Pete Surtees. "Southampton were not in the game first half but they could still have won it in the second, so I was happy that our defence held out the way it did."
But Southampton's Cador was far from cheerful. "The last 20 minutes of the game was the only thing we can take from this game as anything positive," he mused. "We were just not there. We made wrong decisions, did all the things wrong - everything.
"We did exactly what Trojans did to us a few seasons ago against Andover but had we started to play five minutes earlier we would have won. Hats off to Trojans, they deserved it, but we were not very good at all."
Tottonians capitalised on a weary Romsey side, playing their second game in 24 hours, by thrashing them 57-0, scoring nine tries.
The game was as good as over in the opening 15 minutes as the visitors flew into a 26-0 lead but Romsey came back into the game, restricting the London Three South-West high-flyers to just the one further score before the break.
Winger Mike Dibden scored an excellent hat-trick while Joe Jarvis and the outstanding Lee Brading both helped themselves to a couple each in the muddy conditions of the Sports Centre.
But the triumph was marred by a calf injury to Totts skipper Miles Northover, who was taken to hospital and could be out of action for a couple of weeks, missing the crucial league encounter with Effingham & Leatherhead next week.
"We just did the correct things today," said vice-captain Paul Goodall, who kicked six conversions. "But it was difficult as Romsey did kill the ball a lot of the time and prevented us from doing a lot of things we wanted."
Guernsey produced a surprise result on paper, ditching US Portsmouth 16-5, but the Services side was largely experimental and missing a number of regulars on a Navy skiing competition exercise.
And the closeness of Farnborough's 24-32 defeat to Petersfield is also deceptive with the east Hampshire men putting out a scratch outfit, made up of a mixture of first and second team players.
However, the team from the north of the county gave a superb account of themselves and came close; Petersfield leaving it very late before clinching their trip to Tottonians in the semi-finals.
"Farnborough came hard at us," said 'Fields director of rugby Mike O'Shea, "but in the end, we are fairly pleased to have come through the game, considering we had our club pantomime last night in a mix-up with dates and the players were feeling a little rough."
The home side's skipper Alex Stewart felt his side were outstanding - winger Stu Oakley crossing twice. "It was a good result for us and we should have won it but a few changes late on tipped it their way, which was a shame."
Semi-final draw: Tottonians v Petersfield, Trojans v Guernsey.
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