This is Anand's sixth loss to American in Classical chess and the Indian ace will now have to do a lot of hard work to catch up in the last five rounds.
The Indian’s defeat was Nakamura's gain as he moved to the leaders' pack following another draw-marred day that produced just one decisive game.
Out of the 20 games played so far in the Classic, only three have ended decisively.
Anish Giri of Holland, Maxime Vachier-Lagrave of France and Nakamura share the top spot on 2.5 points each near the half-way mark in this US $30000 prize-money tournament.
With five rounds still to come, Anand slipped to ninth in the ten-player round-robin event but is only a full point behind the leading pack.
Anand lost out of a Catalan opening with black pieces. Nakamura obtained a slight advantage out of the opening and Anand's pawn sacrifice did not yield desired compensation although his position remained playable for a long time.
Once Anand's queen got stuck on the edge of the board, the Indian could only exchange it but the resulting endgame was simply much worse. The game lasted 42 moves.
Speaking about the game and his victories against Anand, Nakamura said, "I don't think my style is that much different from Vishy's but I seem to find a way against him."
World champion Magnus Carlsen's hunt for an elusive victory continued well into the fourth round as the Norwegian was held to a draw by Michael Adams of England.
Carlsen pushed for a long time with white pieces out of a Reti opening but Adams put up stiff resistance despite being an exchange down.
Bulgarian Veselin Topalov missed out on an opportunity for his first victory when he let Fabiano Caruana of United States off the hook. Topalov fell to a psychological ploy in a clearly superior position in the middle game.
Results, Round 4: Hikaru Nakamura (US, 2.5) beat V Anand (Ind, 1.5); Magnus Carlson (Nor, 2) drew with Michael Adams (Eng, 2); Anish Giri (Ned, 2.5) drew with Levon Aronian (Arm, 2); Alexander Grischuk (Rus, 2) drew with Maxime Vachier-Lagrave (Fra, 2.5); Veselin Topalov (Bul, 1) drew with Fabiano Caruana (US, 2).
Image: Viswanathan Anand in action at the London Chess Classic tournament.
Photograph: Carl Court/Getty Images
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