LONDON -- For two sets in Wimbledon's fourth round Monday, defending champion Novak Djokovic was in serious trouble, unable to solve 6-foot-8 Kevin Anderson's big serve.
Djokovic dropped both of those sets 7-6 (6), 7-6 (6), then began working his way back into the match. He took the next two sets 6-1, 6-4 to force a fifth set before play was suspended a little after 9 p.m. because of darkness.
The two men will return to Court 1 at 8 a.m. ET Tuesday and play one set that will determine whether the No. 1-seeded Djokovic of Serbia reaches his 25th consecutive Grand Slam quarterfinal or the 14th-seeded Anderson, of South Africa, wins a fourth-round match at a major for the first time in seven tries during his career.
The top seed hasn't lost before the quarterfinals since Lleyton Hewitt did so in the first round in 2003.
Wimbledon is the only Slam that schedules all 16 men's and women's fourth-round matches on the same day, and 15 were completed this time.
Reigning US Open champion Marin Cilic, who beat the last American man in the field, wild-card entry Denis Kudla, 6-4, 4-6, 6-3, 7-5, will need to wait to find out whether he will face Djokovic or Anderson in Wednesday's quarterfinals.
No U.S. player has reached the quarterfinals since Mardy Fish last did it in 2011.
Seven-time champion Roger Federer is back in the quarterfinals at the All England Club. So is 2013 champion Andy Murray.
Federer beat 20th-seeded Roberto Bautista Agut 6-2, 6-2, 6-3 on Monday on Centre Court, putting him in the quarterfinals at the grass-court major for the 13th time.
Federer, who lost in last year's final to Djokovic, needs one more Wimbledon title to break the record he holds with Pete Sampras and 1880s player Willie Renshaw.
He was almost flawless on serve, losing only three points on his first serve and five on his second serve as he improved to 77-9 all-time at Wimbledon -- second only to Jimmy Connors' 84 wins during the Open era.
"I got things figured out at the moment," Federer said.
A possible semifinal showdown could loom against Murray, who made it to the quarterfinals for the eighth straight year with a 7-6 (7), 6-4, 5-7, 6-4 victory against Croatia's Ivo Karlovic on Centre Court.
Karlovic served 29 aces, but Murray broke twice and had only nine unforced errors.
Murray became the first British man to the win the Wimbledon title since Fred Perry in 1936 when he beat Djokovic in the final two years ago. He also won the US Open title in 2012 but lost to Djokovic in this year's Australian Open final.
Murray will next face unseeded Canadian Vasek Pospisil, who came back from two sets down to beat Viktor Troicki of Serbia 4-6, 6-7 (4), 6-4, 6-3, 6-3.
It was the second consecutive five-set triumph for Pospisil, who became just the third Canadian man to reach the quarterfinals at Wimbledon -- joining Robert Powell (1908, '10, '12) and Milos Raonic (2014).
French Open champion Stan Wawrinka also advanced to the quarterfinals, where he will face 21st-seeded Richard Gasquet of France.
Wawrinka, who also reached the quarterfinals last year, beat David Goffin of Belgium 7-6 (3), 7-6 (7), 6-4 on Court 1. The fourth seed from Switzerland is the only player on the men's side not to have dropped a set, winning all 12 sets through his four matches.
Gasquet, who reached the semifinals at the All England Club in 2007, returned to the quarterfinals after beating Nick Kyrgios 7-5, 6-1, 6-7 (7), 7-6 (6) on Court 2.
Kyrgios appeared to stop trying briefly in the second set, half-heartedly swinging at one of Gasquet's serves and simply walking away as another zipped past.
"If they decide to fine me," Kyrgios said, "they can fine me."
Frenchman Gilles Simon made it to the quarterfinals for the first time at the All England Club, beating 2010 Wimbledon runner-up Tomas Berdych 6-3, 6-3, 6-2.
Simon, the 12th seed, has only once before reached the quarterfinals at a major tournament, getting that far at the 2009 Australian Open. And Wimbledon had been the 30-year-old Simon's worst Grand Slam. He only got as far as the fourth round once before, also in 2009.
The Associated Press and ESPN Stats & Information contributed to this report.