Chelsea planted one foot firmly in the European Cup Winners' Cup semi-finals as manager Gianluca Vialli mugged Real Betis with a masterstroke of tactical kidology and towering Norwegian striker Tore Andre Flo fired in two amazing goals in the first 12 minutes in Seville tonight.

Vialli, who had talked all week about the attributes of the 4-3-3 system he had installed at Stamford Bridge immediately after taking over from sacked Ruud Gullit three weeks ago, stunned the Spaniards by dismantaling it and dropping both himself and co-striker Mark Hughes for the quarter-final first leg clash.

Vialli, three times a European trophy winner with Sampdoria and Juventus in Italy, pulled the wool over the Spaniards' eyes by switching to a 4-4-2 format that paid almost instant dividends.

But after Flo's sensational double strike, some of the gloss was taken off Chelsea's triumph when Spain international striker Alfonso pulled a goal back with a header 43 seconds into the second half.

And four Chelsea players were booked despite Vialli's pre-match warning to tread on egg shells throughout the 90 minutes in the intimidating Benito Villamarin Stadium, including the manager's Italian compatriot Gianfranco Zola after the Bulgarian referee accused him of diving.

Inevitably, as tackles started to flow in the second half, Chelsea's spiky little skipper Dennis Wise was among the bookings - a fifth successive caution for the man who must face a Football Association grilling later this month over his dismal disciplinary record.

Young Chelsea substitute Mark Nicholls was also booked within seconds of replacing Zola 10 minutes from the end, while Frank Sinclair incurred another caution.

But that was just the bad news on a night when Vialli got Chelsea's tactics right and his massive consolation for all the bookings was that none will mean suspension for the second leg in two weeks' time.

Beanpole Flo had Chelsea walking tall right from the start. Almost as soon as Dutch goalkeeper Ed De Goey had dived on an under-hit 15-yard drive by Nigerian star Finidi George, Flo was breaking away at the other end onto a short pass by Roberto Di Matteo.

As the Betis defenders backed away from his charge down the wing, he clicked into a higher gear and entered the penalty area before drilling an unstoppable low shot across goalkeeper Prats which nestled neatly in the far corner of the net.

The 3,000-plus Chelsea fans, most of them penned into one corner of the ground and cordoned off from the home supporters by a fixed line of police, could hardly believe it.

And three minutes later they were rubbing their eyes again with a mixture of joy and incredulity as Flo, collecting another short ball from Dan Petrescu which midfielder Olias should have cut out, picked his way delicately and seemingly effortlessly through the centre before hitting another scoring shot that kept superbly low.

The job seemed done already and although Betis were busy and eager in attack, they repeatedly ran into the solid Chelsea rearguard in which Frank Lebouef was outstanding.

Goalkeeper De Goey had trouble with a couple of curling free kicks from dead-ball specialist Robert Jarni and flapped at a corner by Betis skipper Alexis. But Chelsea's tackling in front of him was crisp and decisive until the temperature hotted up in the second half.

Betis were able to raise their game on the back of Spain international Alfonso's close-range header from a right-wing cross by substitue Marquez straight after the break.

And De Goey was happy to see a couple of flying headers by dangerous Alfonso flash just wide.

But against all that, Frank Sinclair had a header cleared off the line and Mark Hughes, making a late entrance in place of hard-running Flo five minutes from the end, should have put away a great chance to make it 3-1.

In the end, however, Flo's 11th goal in just 15 starts this season was enough to carry Chelsea back to Stamford Bridge with a crucial lead thanks to De Goey's brilliant save from yet another Alfonso header right at the death.

Real Betis: Prats, Merino, Alexis, Camas (Oli 65), Alfonso, Jarni, Solozabal, Olias, Vidakovic (Marquez 45), Fernando, Fenidi.

Subs Not Used: Pinto, Luis Fenendez, Perez, Quelar, Josepe.

Booked: Fernando, Solozabal, Jarni. Goals: Alfonso 46.

Chelsea: De Goey, Petrescu, Leboeuf, Duberry, Clarke, Wise, Di Matteo, Flo (M. Hughes 85), Sinclair, Newton, Zola (Nicholls 79).

Subs Not Used: Hitchcock, Lambourde, Vialli, P. Hughes, Charvet.

Booked: Zola, Wise, Sinclair, Nicholls.

Goals: Flo 9, 12.

Att: 31,000

Ref: A Ouzounov (Bulgaria).