Chelsea and Real Madrid have landed world-class players via prudent transfer acitivity. It seems another could be on the way this summer, with Los Blancos said to be monitoring one of the Blues' key defenders.According to reports, the Spanish giants have added Chelsea center-back Antonio Rudiger to their wishlist as Carlo Ancelotti eyes defensive reinforcements. The defender has emerged as a shocking target for Real Madrid, who are badly short of options at the heart of the defense.Antonio Rudiger will weigh up his options before deciding whether to sign a new Chelsea contract. Real Madrid and PSG are both keeping an eye on the situation.- @SkySportsNews— Simon Phillips (@siphillipssport) August 11, 2021Recall that Los Blancos have a huge gap to fill in the backline following the departure of Sergio Ramos this summer. The Spaniard left the Santiago Bernabeu for a switch to Paris Saint-Germain a few weeks ago and is yet to be replaced.Raphael Varane has also completed his long-anticipated transfer to Manchester United, leaving Real Madrid without their favored canter-back pair. Los Blancos have less than two weeks to sort out their centre-back crisis as the transfer window shuts by the end of the month.0.4 - Chelsea have conceded 26 goals in 19 Premier League games when Antonio Rüdiger hasn’t started this season (1.4 per game), compared to just seven conceded in the 17 games when the German has started (0.4 per game). Loss. pic.twitter.com/uZYUgTLjJW— OptaJoe (@OptaJoe) May 12, 2021Meanwhile, Antonio Rudiger continues to be a vital figure in Chelsea's squad. The German put in plenty of rock-solid performances for the Blues last season as they made history by winning the Champions League for the second time. He made 34 appearances for the club across all competitions, scoring one and setting up another.The Chelsea center-back has emerged as shocking target for Real MadridReal Madrid's summer in a nutshellThe current transfer window has been a strange revelation for many Real Madrid fans. Plagued by an ardous financial situation, the club has been forced to forget their 'Galacticos' tradition and face the reality of the post-COVID-19 economy.So far, there have been more notable departures and arrivals at the Santiago Bernabeu. Apart from Sergio Ramos and Raphael Varane, Brahim Diaz and Takefusa Kubo also went on loan to AC Milan and Mallorca respectively.The only major addition to the squad is David Alaba, who arrived as a free agent from Bayern Munich. Meanwhile, Los Blancos will be boosted by the return of players like Gareth Bale, Martin Odegaard and Dani Ceballos from their respective loan stints.