The world champions, Germany, reached the Confederations Cup semi-final on Sunday with a 3-1 victory over a Cameroon side reduced to 10 men after further confusion with experimental video replays.

Germany were leading through Kerem Demirbay’s strike when a case of mistaken identity halfway through the second half – which replays were meant to eradicate – left Cameroon bemused and irritated.

Sebastien Siani was dismissed for a foul on Emre Can after referee Wilmar Roldan inspected a pitch-side monitor. It then took furious protests from the Cameroon players, including Siani sarcastically applauding Roldan, for the Colombian to check the monitor again and discover he had sent off the wrong player. Siani was called back and Ernest Mabouka was correctly sent off.

“The referee did not give any explanation,” Cameroon coach Hugo Broos. “It’s a foul, fair enough but both players had their legs high up to reach the ball. I don’t understand why only my player received a red card.”

Germany extended their lead two minutes after the red card courtesy of a diving header from Timo Werner. There were signs of a comeback by Cameroon when Vincent Aboubakar’s flicked header slipped through Marc-Andre Ter Stegen’s raised hands. But Werner scored again in the 81st minute to secure a 100th victory for Joachim Löw in his 150th match in charge of Germany.

By securing the top spot in Group B, an inexperienced Germany squad playing without many of its 2014 World Cup winners will now stay in the Black Sea resort of Sochi and play Mexico on Thursday. The other semi-final on Wednesday sees Portugal play Chile, who finished second in Group B after drawing 1-1 with Australia in Sunday’s other game.

The Germans will have to display greater urgency against Mexico than they did in the first half against Cameroon. Löw’s side was wasteful, with wing back Joshua Kimmich sending a diving header wide, and were largely restricted to attacking down the flanks. Ter Stegen prevented Cameroon going in front, with the goalkeeper’s one-handed save stopping Andre-Frank Zambo Anguissa’s hooked shot across goal at the end of a nervy first-half for the Germans.

“Some of my players have no experience and were nervous because one mistake could have eliminated us,” Löw said. “It’s good to see how we were able to ... up the ante.”

The Germans opened the scoring with a slick move. Julian Draxler fed Demirbay with a back-heel through Siani’s legs and the midfielder marked his first international start with a shot into the top corner with his weaker right foot. Cameroon goalkeeper Fabrice Ondoa prevented Germany from extending their lead by blocking Kimmich’s shot from a tight angle.

Then came the red-card confusion, swiftly followed by Germany’s second goal. Werner evaded Adolphe Teikeu to meet a cross from Kimmich and open his international account with his stooping header.

The blunder by Ter Stegen for Cameroon’s goal should concern Löw. It was the latest by a Germany goalkeeper in the World Cup warm-up tournament. Bernd Leno was also at fault for Australia’s two goals in Germany’s 3-2 victory in their Group B opener.

But Werner restored Germany’s two-goal cushion. The striker whose goals propelled Leipzig to a surprise second-place finish in the Bundesliga turned a cutback from Benjamin Henrichs into the net. “He has a killer instinct,” Löw said.

Cameroon leave Russia early and might not be back next year, given their struggles in qualifying. “We are one of the best teams in Africa but there is still a difference between us and modern football,” Broos said. “We see the proof of that, that there’s a lot still to be done.”