New Delhi: Tournament favourite Viswanathan Anand of India suffered a shock defeat at the hands of defending champion Rustam Kasimdzhanov of Uzbekistan in the fourth round of the World Chess Championship at San Luis, Argentina.

The loss proved expensive for Anand as he lost the joint lead and was relegated to third spot in the $1 million event being played on a double round robin basis among eight players, according to a statement received here.

Much like the previous round, the fourth round provided results in all the four encounters and Veselin Topalov of Bulgaria ground out a win over Michael Adams of England in 30 moves to go into sole lead with 3.5 points.

Russian Peter Svidler got his second straight victory, beating compatriot Alexander Morozevich in an interesting King's Indian game.

Peter Leko beat fellow Hungarian Judit Polgar in 25 moves of a Sicilian Paulsen and improved his position after being at the bottom after three rounds.

In a sharp and risky line in the Najdorf, Anand faced the English attack late Saturday night and seemed to be in trouble early as Kasimdzhanov exploited the imbalance in his rival's side.

He got an advantage in the middle game as Anand played riskily and got into a wrong plan. The Uzbek attacked Anand's weakened king and the Indian star resigned after the 38th move.

Topalov leads the current standings with 3.5 points after four rounds and Svidler took his tally to three points with Anand at sole third with 2.5 points.

Kasimdzhanov moved up in the standings to fourth position with two points while Hungarians Polgar and Leko are next with 1.5 points each.

Morozevich and Adams are at the bottom of the table with one point each.

Results (Round 4): Leko beat Polgar; Kasimdzhanov beat Anand; Topalov beat Adams; Morozevich lost to Svidler.

Standings (After Round 4): 1. Topalov 3.5 points; 2. Svidler 3; 3. Anand 2.5; 4. Kasimdzhanov 2; 5. Polgar and Leko 1.5; 7. Morozevich and Adams.

Pairings for Round 5: Adams vs. Kasimdzhanov; Anand vs. Leko; Polgar vs. Morozevich; Svidler vs. Topalov.