Vincent Keymer waited and pounced on that one mistake to subdue world No. 1 Magnus Carlsen, even as Wesley So and three others teetered on the brink of elimination in the opening game of Round 4 of the FIDE World Cup Wednesday in Baku, Azerbaijan.

Keymer, the prodigious 18-year-old from Mainz, Germany, who completed grandmaster status five years ago, parried all attacks by Carlsen in a seemingly dull knights-and-pawn drawish endgame. That lone error by Carlsen allowed Keymer to capture a single pawn and nursed it to victory in their Queen's Gambit Declined game.

Carlsen gets to play the white pieces and must win at all costs in the second game of the best-of-two games eliminator using classical match time controls.

He gets to pack his bags back to Norway if Keymer manages to find drawing lines.

Just a draw will advance Keymer outright into the Final Eight boards, like Round 4.1 winners Alexey Sarana, Santos Gujrati Vidit of India, Nils Grandelius of Sweden and Ferenc Berkes of France. European Individual championships title holder Sarana punished the erratic handling of the Queen's Gambit Declined game by the white-playing So, the eighth seed Filipino-American.

Sarana, the Russian-born 23-year-old playing under the Serbian flag, found the right combinations in punishing So's back-to-back blunders on the 27th and 29th moves that allowed him to launch a massive attack with a knight, bishop, rook and a pawn on the h-file and the win five moves later.

Ferenc Berkes of France beat former world Top 10 elite Ruslan Ponomariov of Ukraine. Vidit, one of four Indians who have advanced thus far, demolished Etienne Bacrot of France.

Nils Grandelius of Sweden was brilliant in the endgame win against Jaime Santos Latasa of Spain.

Second seed Hikaru Nakamura and third seed Fabiano, Caruana of the US, and fourth seed Ian Nepomniatchi of Russia, playing under the FIDE banner, were held to fighting draws like 10 other matches.

Defending champion Jan-Krzysztof Duda of Poland and Parham Maghsoodloo of Iran essayed the Ruy Lopez Closed variation and declared a truce in 31 moves.