Xabi Alonso is unsure if Kylian Mbappe will be fit enough to start when Real Madrid take on Borussia Dortmund.

Mbappe is yet to make a start at the Club World Cup, having missed several matches due to illness.

The France star returned to action as Madrid saw off Juventus 1-0 on Tuesday in the last 16.

Mbappe replaced Gonzalo Garcia, who has starred in the 26-year-old's absence. After netting Madrid's goal in that win over Juve, he became the only player to directly contribute to a goal in all of his team's matches at this edition of the Club World Cup.

And with Garcia in such rich form, Alonso is not going to throw Mbappe in from the start unless the forward is absolutely ready.

"He's getting better. He's been improving over the last three days," Alonso said.

"[On Saturday] we will be deciding if he will be part of the initial XI."

Asked if he could partner Mbappe and Garcia, Alonso replied: "I think they could complement each other and if it's something that is required, it's not something I'm going to turn a blind eye to."

Saturday's clash with Dortmund will be Madrid's 20th match at the Club World Cup (including previous formats), and they are yet to be defeated in the competition (W16 D3 L0).

Since 2014, Madrid have won 14 of 15 Club World Cup matches, only failing to do so against Al-Hilal in the group stage this year (1-1).

Madrid have won four straight meetings with Dortmund, all in the Champions League, including a 5-2 victory in the league phase last October. Prior to this run, which began in 2017, Los Blancos had recorded just four wins in 12 encounters with the German giants (D5 L3).

"Dortmund is a very good team. As soon as [Niko Kovac] was at the helm, they were growing and developing and on the rise," added Alonso.

"We are facing a tough opponent. It's going to be tight tomorrow."