Novak Djokovic, Roger Federer, Andy Murray and Stan Wawrinka can justify their status as Wimbledon's big four by making the semi-finals today. Defending champion, top seed and 2011 winner Djokovic takes on US Open champion Marin Cilic boasting a 12-0 career lead over the Croatian. Seven-time champion Federer faces Gilles Simon with the Frenchman playing in his maiden Wimbledon quarter-final while 2013 winner Murray, seeded three, is up against surprise quarter-finalist Vasek Pospisil of Canada. This was one of the most difficult matches I've played at Wimbledon, maybe in my career. At times, I was really helpless with my return Novak Djokovic Fourth seed Wawrinka tackles 2007 semi-finalist Richard Gasquet, the second Frenchman in the last-eight. Djokovic survived a barrage from big-hitting South African Kevin Anderson to secure his seventh successive Wimbledon quarter-final yesterday. The top seed came from behind to win 6-7 (6-8), 6-7 (6-8), 6-1, 6-4, 7-5 in a last-16 tie that had resumed yesterday after being suspended due to bad light on Monday at the end of the fourth set. "I was two sets down. To come back and win in five definitely gives me great satisfaction and confidence for the next challenge," Djokovic said. "It was frustrating at times, especially today, but I managed to go through, and that's what matters." Djokovic struggled throughout the match in trying to tame the 2.03-metre Anderson's huge serve, but broke him to go up 6-5 in the fifth set to set up his victory. "Kevin served exceptionally well," Djokovic said. "I find that this was one of the most difficult matches I've played at Wimbledon, maybe in my career. At times, I was really helpless with my return." Anderson finished with 40 aces, including three in the first game of the fifth set and eight overall in the decider. In the women's draw, Garbine Muguruza's new-found love-affair with grass blossomed further, as she became the first Spanish woman in 18 years to reach the semi-finals with a 7-5, 6-3 win over Timea Bacsinszky. Maria Sharapova is also back in the semi-finals for the first time since 2011. The fourth-seeded Russian struggled for a bit, but still beat CoCo Vandeweghe of the United States 6-3, 6-7 (3-7), 6-2. Cilic, 26, lost to Djokovic in five sets in the quarter-finals in 2014 despite being two sets to one ahead. He then went on to win a maiden grand slam title at the US Open in September. "I know him very well. We played many, many matches on different surfaces," said Djokovic after reaching his seventh successive Wimbledon quarter-final and 25th in a row at the majors with a 6-7 (6-8), 6-7 (6-8), 6-1, 6-4, 7-5 win over Kevin Anderson of South Africa. "I remember him mentioning earlier this year that he wants to focus mostly on Wimbledon, grass. That's where he feels like he has a best chance besides US Open. He's playing at a very high level." Cilic admitted he needs to match Djokovic blow for blow today. "When Novak is playing at the speed that he's playing, I'm going to have to be more aggressive to play more risky tennis," said Cilic. Agence France-Presse, Reuters, Associated Press