Magnus Carlsen, the world champion, made the most embarrassing blunder of his career at Tata Steel Wijk aan Zee last weekend, missing a chance for a simple virtually forced mate in three moves, all of them rook checks. That game was drawn and the next day he lost after a series of errors. His lead in the official rankings, which the 26-year-old Norwegian says he prizes even more than the global crown, has shrunk drastically and is threatened by two American rivals.

This weekend’s crucial games at Wijk and at Tradewise Gibraltar could therefore be a defining moment for Carlsen. Only seven players, starting with the legendary Bobby Fischer, have been No1 since the official Fide list was instituted in 1971 and Carlsen’s tenure of the top spot has been unbroken since July 2011. However, in his last 23 classical games, the world title series against Sergey Karjakin in New York and the first 11 rounds at Wijk, he has only managed +4=17-2, well short of the dominance expected from a great champion.

Carlsen plays the two final rounds at Wijk while the world No2, Fabiano Caruana, is in action at Tradewise Gibraltar – the most important Open on the European calendar. The world No3, Wesley So, was half a point clear at Wijk after round 11 of 13 and the rating points gap between him and the top pair is narrowing. So’s unbeaten run in major events now extends to more than 50 games.

China’s champion and prodigy Wei Yi is the sensation of Wijk. The 17-year-old has played sparkling attacks, beat Karjakin in round 11 and advanced to clear second place half a point behind So. Leaders with two rounds to go were So 7.5/11, Wei Yi 7, Carlsen, Levon Aronian (Armenia) and Pavel Eljanov (Ukraine) all 6.5.

Carlsen’s critics say his increased fallibility, seemingly due to lapses in concentration and poor psycholoigcal reactions to defeat, began in 2014 when an adoring home public expected too much from their hero at the Tromso Olympiad. NRK, Norway’s BBC, televises all his major competitions live and this means extra pressure. He missed a couple of easy wins in the Karjakin match through playing fast and the same occurred at Wijk where, with plenty of time on the clock, he made his blunder against Anish Giri after just a few seconds’ thought. The ending became drawn but Carlsen played it out for 120 moves until stalemate. Next day he was tired and rattled, and was crushed by the 21-year-old Hungarian talent Richard Rapport.

Carlsen’s Wijk tournament and his No1 ranking can still be secured if he is in his best form this weekend, while Caruana and So will also be striving for strong finishes. It should make for two fine afternoons of online chess viewing, with the key games from both events free and live to watch online along with move-by-move computer and grandmaster commentaries which make the action easy to follow for the ordinary amateur.

England’s Gawain Jones is in second place, half a point behind Jeffrey Xiong of the US, in the B group whose winners qualifies to take on Carlsen and the elite in 2018.

Aronian’s creative attack below was the brilliancy of the event. His shrewd 9 Be1 varied from a previous Richard Rapport game and provoked 9...Re8? when d6 or d5 were better. After that 15 Ng5! 19 exf5! and 21 f4! opened up Black’s defences for White’s queen and rooks and the winning sacrifice 27 Rxf8+! At the end if Kg8 29 Rxg7+ soon mates.

Levon Aronian v Richard Rapport, Wijk 2017

1 d4 Nf6 2 c4 e6 3 Nf3 b6 4 g3 Bb7 5 Bg2 Be7 6 O-O O-O 7 Nc3 Ne4 8 Bd2 Bf6 9 Be1!? Re8? 10 Qc2 d5 11 Nxe4 dxe4 12 Nd2 Bxd4 13 Rd1 Qc8 14 Nxe4 Bc5 15 Ng5! f5 16 Bxb7 Qxb7 17 Bc3 Bf8 18 e4 h6 19 exf5! hxg5 20 f6 c5 21 f4! g4 22 f5 gxf6 23 fxe6 Qh7 24 Qg2 Na6 25 Rd7 Qh5 26 Rxf6 Rad8 27 Rxf8+! Kxf8 28 Qf1+ 1-0

3479 1 c4! wins a piece after Nb4/Ne7 2 c5! Bxb3 3 Qxb3+ and 4 cxb6, or Nf4 2 Bxf4 exf4 3 Rxe6.