Cristiano Ronaldo, Robert Lewandowski will decide Champions League semifinal - Xabi Alonso

Xabi Alonso said the Champions League semifinal between his former sides Real Madrid and Bayern Munich could be decided by the strikers.

Madrid's Cristiano Ronaldo travels to Wednesday's first leg in Munich having found the net in all 10 Champions League games this season, and his hat trick was largely responsible for knocking out Bayern in last season's quarterfinals.

Bayern striker Robert Lewandowski has 18 goals in 17 games so far in 2018, and he had a memorable four-goal haul against Madrid during the 2012-13 Champions League semifinal in which he was playing for Borussia Dortmund.

"Normally, when Madrid and Bayern face each other, it is in a big occasion," Alonso said. "For both clubs, this semifinal is a great chance. At this stage, you cannot afford to have a weakness. You need to be a unit, and the key players, especially the strikers, they need to make a difference. Whichever side's strikers have the biggest impact will get a big advantage. I think it is quite balanced at the moment."

Both teams need to keep things tight in defence, and considering Ronaldo's ability to score, Bayern's back line must focus, Alonso said.

"Madrid are going to have to suffer in some moments, and Bayern must be careful with Madrid's attacks," he said. "Madrid need half a chance to score a goal, and their players get forward a lot, especially Cristiano Ronaldo, who is in a state of grace."

Alonso said Arturo Vidal's absence through injury was a blow for Bayern, but he believes the Bundesliga side has the squad depth to cover.

"[Vidal's injury] is a problem for sure, but I am sure they will deal with it," Alonso said. "Arturo is a top player. He delivers the best of himself in these big clashes. I am sorry for him as I know how much he loves to play these kind of games, but I am sure that Jupp Heynckes will take the best decision to deal with his injury. That cannot be an excuse."

Alonso won six trophies during five years at Madrid, including the 2014 Champions League, before moving to Bayern in 2014 and winning the Bundesliga title in each of his three seasons in Munich.

He said it was impossible to say in advance whether having the second leg at home was an advantage for Madrid.

"Whenever you have the chance to play the second leg away, each goal is crucial, but at the same time, you want the support from your fans," Alonso said. "You never know. I couldn't say which is best."