Viswanathan Anand was dealt a stunning defeat by Lev Aronian in the fifth round of the Morelia-Linares chess tournament.
The Indian ace, who was in sole lead after winning his previous two games, slipped to tied second after the defeat.
After two more rounds, the action will shift to Linares.
In next two rounds, Anand plays Vassily Ivanchuk and Peter Leko.
The 16-year-old Magnus Carlsen, who lost to Anand in third round, scored an upset win over Veselin Topalov to take sole lead at 3.5 points after five rounds.
Aronian and Anand have three points each and are in tied second place.
On a day of shocks, both the top seeds lost, Anand and Topalov lost in tense struggles. Anand, with black pieces in a Queen's Gambit Accepted, resigned in 48 moves. It was his first loss in the tournament.
This was also Anand's first-ever loss to Aronian, who he has beaten once in the past, in the Melody Amber blindfold game last year. Four other games between the two were drawn.
Meanwhile, Topalov resigned against Carlsen, when it seemed he had drawing chances. Carlsen himself showed Topalov the defence after he resigned.
In other games, Morozevich drew with Vassily Ivanchuk and Peter Svidler drew with Peter Leko.
In round four, Anand beat Morozevich in the Breyer System of the Ruy Lopez. After seeming to hold is own earlier, Morozevich started to drift around the 40th move and then his 41st move saw him slip badly. Anand finished him to grab the full point.
Results of Round 5:
Aronian beat Anand; Svidler drew with Leko; Morozevich drew with Ivanchuk; Carlsen beat Topalov.
Placings after Round 5: 1. Carlsen 3.5; 2. Aronian and Anand 3.0; 4. Svidler, Leko and Ivanchuk 2.5; 7. Topalov and Morozevich 1.5.
The moves - Anand Aronian
1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.Qc2 dxc4 5.Qxc4 Bf5 6.g3 Nbd7 7.Nc3 e6 8.Bg2 Be7 9.0-0 0-0 10.Re1 Ne4 11.Qb3 Qb6 12.Nh4 Bxh4 13.gxh4 Nef6 14.e4 Bg6 15.Qxb6 axb6 16.Bf4 Rfe8 17.Rad1 b5 18.Bd6 e5 19.d5 Nh5 20.Bf1 f6 21.b3 Nf4 22.a4 bxa4 23.bxa4 Bf7 24.Rb1 Ra7 25.Red1 Rc8 26.Ne2 Nxe2+ 27.Bxe2 cxd5 28.exd5 Nf8 29.Bb5 Raa8 30.Be7 Ng6 31.d6 Nxe7 32.Bd7 Nc6 33.Rxb7 Nd4 34.Bxc8 Rxc8 35.Rdb1 Rf8 36.Rb8 Be8 37.a5 Nf3+ 38.Kf1 Nd2+ 39.Ke1 Nxb1 40.a6 Bc6 41.a7 Kf7 42.d7 Ke7 43.Rxf8 Kxd7 44.a8Q Bxa8 45.Rxa8 h5 46.Ra7+ Ke6 47.Rxg7 Kf5 48.Rg3 1-0.