World Champion Viswanathan Anand was stunned by Veselin Topalov of Bulgaria in the fourth round of the Bilbao Grand Slam final chess tournament in Bilbao, Spain.
On a day that provided all three decisive games in this category 22 event, 17-year old Magnus Carlsen of Norway became the unofficial world number one after beating Teimour Radjabov of Azerbaijan.
The other game of the day saw Armenian Levon Aronian striking form as he defeated Vassily Ivanchuk of Ukraine in a long drawn affair.
Former world champion Topalov retained his full point lead over Carlsen in the experimental football-like scoring system that gives three points for a win and one for a draw.
With six rounds remaining for the fight of Euros 150000 first prize, the Bulgarian has moved to 8 points. Aronian stands third on 5, two points clear of nearest rivals Radjabov, Ivanchuk and Anand, all of whom yet to register a single victory.
Anand was tamed by Topalov in a Queen's Indian defense game where the Indian played black. Topalov went for a popular set-up involving a pawn sacrifice early in the opening and the Indian's passive play resulted in a huge spatial advantage to white.
Topalov was a man on a mission after his previous round victory over Carlsen and guided his pieces quite well to launch a dangerous attack.
The position was already much inferior when Anand found his queen trapped on the first rank and called it a day. Topalov needed just 25 moves to topple the reigning world champion.
The loss proved costly for Anand as he temporarily lost his numero uno position in the rating list and for the first time ever, Carlsen became the number one.
However, these figures are unofficial yet as the net calculations will be made only after the tournament completes.
While Topalov displayed top form and high-class chess, a resurgent Carlsen scored comprehensive victory over Radjabov who employed the Sicilian Dragon as black.
Playing according to the position, Carlsen launched a king side attack and tackled Radjabov's pieces effectively on the other flank.
The Azerbaijani was out-of-sort and out-of-form as Carlsen bruised and cruised in 37 moves.
Aronian was the winner in the longest game of the day against Ivanchuk after the latter failed to find the best defensive resources in a slightly inferior endgame.
It was a Queen's gambit accepted in which Ivanchuk was saddled with an isolated pawn in the endgame and Aronian wove a brilliant web on both flanks to show his class.
Ivanchuk lost a couple of pawns and fought in vain before resigning on the 70th move.