Schalke's 2-0 win at home to FC Basel on Wednesday confirmed their place in the knockout stages of Europe's leading club competition, but Keller is still living in fear of the sack having been told his future will be decided during the Bundesliga's winter break.

Keller is due to celebrate a year in office on Monday, but defeat at home to third-from-bottom Freiburg could be his last game in charge with his team currently seventh in the table.

"I can't say anything different after each game, whether we win or lose," said Schalke's general manager Horst Heldt on the subject of Keller's future.

"It remains the case that we will get together after the Nuremberg game (on December 21) and take stock."

Heldt has promised a "merciless" appraisal of Keller's performance, but it is clear the 43-year-old can ill afford further defeats, especially as Schalke's last two opponents of 2013 are currently in the bottom three.

Having had to beat Basel to stay in the Champions League, Keller's relief at reaching the last 16 was palpable and he needs another win at the Veltins Arena on Sunday to improve his chances of staying in charge.

"No one can understand the sort of pressure the team and I were under," he said.

"There was tremendous joy at the final whistle, no one had given us any credit.

"It's enormously important to reach the last 16, especially for the finances.

"Now we have to put up 11 players against Freiburg."

Keller was referring to a mounting injury list as both captain Benedikt Hoewedes and attacking midfielder Julian Draxler will not play again this year after picking up hamstring injuries against Basel.

Long-term knee-injury victims Dennis Aogo, Klaas-Jan Huntelaar and Marco Hoeger are all out until at least early next year.

Schalke's erratic form has seen them leak 28 goals in 15 Bundesliga games this season and centre-back Felipe Santana says the Royal Blues are wary of suffering a surprise defeat.

"Freiburg have many qualities, they are a dangerous opponent," said the Brazilian.

"We need to be highly concentrated for the full 90 minutes to avoid suffering a terrible surprise."