A second-half hat-trick by Billy Sharp helped Saints come from behind to record a 4-1 win over Swiss side Étoile Carouge this evening.
Nigel Adkins' men trailed to the Geneva-based outfit at the break, with a 38th-minute Jos Hooiveld own goal the difference in a keenly-contested opening period.
But Sharp, one of nine half-time substitutes, made the difference after the break with three excellently-taken strikes.
He was preceded in getting on the scoresheet by youngster Luke Shaw, who levelled nine minutes into the second period with a curling effort.
Adkins had named a starting XI including a combination of players from the sides selected for the games against Anderlecht and Arsenal last weekend.
Kelvin Davis started between the sticks, behind a defence made up by Danny Butterfield, skipper José Fonte, Hooiveld and Danny Fox.
Saints' three-man central midfield consisted of Morgan Schneiderlin, James Ward-Prowse and Adam Lallana, while frontman Rickie Lambert was supported by Guly and Jason Puncheon.
The game's first opportunity went Saints' way as Puncheon found space on the right flank before delivering a tantalising delivery onto the head of fellow wide-man Guly, but his fourth-minute effort hit the side-netting.
The visitors were in the ascendancy for the opening quarter-hour, though Étoile did find an opening against the run of play when a cross from the right fell invitingly for Marco Delley, but his half-volley deflected wide via the offside Benjamin Besnard.
Guly then worked himself another opportunity with some good running down the left flank, but his eventual shot at goal dragged the wrong side of the near post.
The hosts were then handed a decent opportunity when Fonte brought down Delley on the edge of the box, but Oliver Maric fired well over after being teed-up by Besnard.
A ding-dong encounter then led to Saints creating their best chance so far as a superb move ended with the ball breaking for Guly, but he was denied by a great save from Étoile 'keeper Sébastien Roth.
Saints had the ball in the net not long later when Ward-Prowse tapped in after Lallana's shot had been parried, but the youngster was ruled offside by the assistant referee.
Adkins' men continued to turn the screw, with an excellent opportunity falling Puncheon's way on the half-hour when Lambert squared to him in the area but the winger shot over from close range.
But Saints went behind against the run of play seven minutes before the break as Delley's low cross from the left caused difficulty for the defence, leading to Hooiveld heading into his own net.
However, the visitors responded quickly, with good work by Ward-Prowse allowing him to set up Lallana on the edge of the area only for his low effort to flash just wide.
Lallana turned provider soon afterwards as his hanging corner-kick found Lambert, but his header was tipped away from the goal by Roth.
HALF TIME: ÉTOILE CAROUGE 1-0 SAINTS
Adkins made nine changes at the break, introducing Tommy Forecast in place of Davis with a new back four including Frazer Richardson, Jack Stephens, Dan Seaborne and Dan Harding.
Dean Hammond and Calum Chambers came into midfield alongside Ward-Prowse, while Puncheon was joined in attack by Sharp and Shaw.
And it was youngster Shaw who drew his side level as he combined with Chambers before curling inside the far post with his right foot for a 54th-minute equaliser.
The home side responded by making five changes, but Saints continued to grow in confidence - and they could have taken the lead had Sharp's close-range effort not been excellently saved by Roth's legs.
Adkins made his final change midway through the second period when he brought on Richard Chaplow for Puncheon, and Saints were soon in front.
Ward-Prowse - the only player to play the full 90 minutes - took a low free-kick from the left flank that caught the home defence unawares, allowing Sharp to sweep the ball beneath Roth with 20 minutes remaining.
The striker added his second six minutes later when, after Ward-Prowse had seized possession in midfield, Sharp collected the ball via Chaplow before advancing and finishing excellently.
And the frontman rounded off his treble with nine minutes to go as he latched onto Chambers' chipped pass before sliding Saints' fourth inside the far post.
Substitute Chaplow went close to making it five barely a minute later when he met Chambers' pass on the volley, but his shot was superbly palmed away by Roth.
A surging 88th-minute run down the right flank by Richardson then allowed him to square for Ward-Prowse, but the midfielder's poked attempt snuck just wide of the far post.
FULL TIME: ÉTOILE CAROUGE 1-4 SAINTS
Étoile Carouge (4-4-2): Roth; Pauchard (Pasquier 54), Doudet (Araujo 70), Maric (Gross 75), Pedretti (Baumgartner 46); Valente (Soares 54), Ajdini (Pomevor 54), Barroso, Delley (Roch 62); Besnard (Bajrami 62), Carrupt (Casara 67). Subs not used: Ibrahimi.
Goals: Hooiveld OG (38).
Saints (4-3-2-1): Davis (Forecast 46); Butterfield (Richardson 46), Fonte (Stephens 46), Hooiveld (Seaborne 46), Fox (Harding 46); Schneiderlin (Hammond 46), Ward-Prowse, Lallana (Chambers 46); Puncheon (Chaplow 67), Guly (Shaw 46); Lambert (Sharp 46).
Goals: Shaw (54), Sharp (70, 76 & 81)
Referee: Nicholas Jancevski