Kewell had given Leeds the perfect start by putting the Premiership side into the lead after just 14 minutes. However, it became clear that Leeds would be in for a difficult night when Spartak equalised just seven minutes from the break.
The match had been billed as something of a grudge match because of the controversy surrounding the original postponement of the fixture last week.
A dangerous, frozen pitch at the Dinamo Stadium had led to the initial tie being postponed and switched to Levski Sofia's Georgi Asparuchov ground, where conditions were in stark contrast to a week ago.
The playing surface was lush and green, and though temperatures were just beginning to touch freezing when the match kicked off, it was still 20 degrees warmer than in the Russian capital.
A lively local crowd of around 6,000 had also gathered, with the Bulgarians cheering Leeds, for a long-awaited kick-off which finally brought an end to what the United chairman, Peter Ridsdale, had described as "psychological warfare".
Comments by the Spartak coach, Oleg Romantsev, had been the most disparaging, with his tirade against O'Leary and the club likely to land him in hot water with Uefa, the European game's ruling body.
O'Leary, meanwhile, just wanted his players to do their talking on the pitch, and he was rewarded with a performance in the first 30 minutes which spoke volumes for Leeds' season to date.
The Premiership leaders sparkled, despite the absence of two of their most experienced players in David Batty, who has an Achilles injury, and the captain, Lucas Radebe, who was suspended.
It was left to the Republic of Ireland international, Gary Kelly, to lead his side into action, and the right back did so with a cool authority as United were rarely troubled during that opening half-hour.
In truth, Leeds should have had more than the one goal to show for a dominant first-half display against a team who had not played for a month following the end of their league campaign.
Spartak were lucky not to go behind in the sixth minute when Kewell hammered a near-post corner from Ian Harte against the bar.
However, the Australian forward made amends eight minutes later when he latched on to a woeful back pass from Dmitri Khlestov and rounded the Spartak goalkeeper, Alexander Filimonov, before tucking the ball home from an acute angle for his sixth goal of the season.
Michael Bridges was looking stunning on his own up front and had chances either side of Kewell's strike, until the tone of the game turned 10 minutes from the break.
Spartak were looking bereft of ideas until Viktor Bulatov curled in a 20-yard left-foot shot which Nigel Martyn, the Leeds goalkeeper, athletically tipped over the bar.
It seemed to wake up Spartak because within three minutes they were level when Vassili Baranov's corner struck Jonathon Woodgate and kindly fell to Alexander Schirko, who turned sharply to fire past Martyn and score from close range.
The half-time whistle then came as a relief to Leeds and O'Leary, who was suddenly faced with an entirely different opposition to that which had started the game.
The break had given O'Leary the chance to regroup his troops and take stock after ending the opening half under pressure. It was through Kewell that Leeds channelled their early forward aggression, with one jinking run eventually thwarted by Filimonov.
The Australian also surged down the right-hand side of the area at one point, but in running too wide his right-foot drive was over the bar.
The first booking soon followed, with the Swedish referee, Anders Frisk, showing yellow to the Spartak left-back, Yuri Kovtun, who will now miss next week's second leg through suspension.
Shortly after, Bridges - who had required treatment for a knee injury midway through the first half - then limped off in the 55th minute, with Darren Huckerby coming on.
But, despite his great pace, Leeds were soon finding themselves stretched at the back as their marking was beginning to fall apart amid Spartak's neat passing and swift movement.
Corners, free-kicks and one genuine penalty appeal - Duberry bringing down Igor Titov - found Leeds almost at breaking point, which then finally came in the 65th minute.
It was a defining moment as Kewell had again weaved his way into the heart of the Spartak area, with a toe-poked shot saved by the outstretched boot of Filimonov.
Within seconds Spartak were on the attack and a brilliant move ended with Schirko's simple pass - with the Leeds defence exposed - to Robson, allowing the Brazilian to tap home into an empty net.
Spartak Moscow: Filimonov; Kovtun, Khlestov, Bushmanov, Baranov, Bulatov, Titov, Parfionov, Schirko, Bezrodniy, Robson. Substitutes: Smetanin (gk), Ananko, Ketchinov, Evseyev, Meleshin, Mor, Everton.
Leeds United: Martyn; Kelly, Woodgate, Duberry, Harte, Bakke, Bowyer, McPhail, Haaland, Kewell, Bridges. Substitutes: Robinson (gk), Mills, Maybury, Jones, Huckerby, Smith, Hiden.
Referee: A Frisk (Sweden) .Reuse content