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2008: Illness, Olympic gold, and fifth US Open

Federer's success in 2008 was severely hampered by a lingering bout of mononucleosis, which he suffered during the first half of the year. At the Australian Open Federer lost in the semifinals to eventual winner Djokovic, which ended his record of 10 consecutive finals. He lost twice in Masters finals on clay to Nadal, at Monte Carlo and Hamburg. Federer captured three titles playing in 250-level events at Estoril, Halle, and Basel. Federer was defeated by Nadal in two Grand Slam finals, the French Open and Wimbledon, which was regarded as the best match of tennis history by many, when he was going for six straight wins to break Björn Borg 's record. He came back from two sets down to force a fifth set, where he fell just two points from the title.

At the Olympic Games, Federer and Stan Wawrinka won the gold medal in doubles by beating the Bryan brothers American team in the semifinals and the Swedish duo of Simon Aspelin and Thomas Johansson in the final. However, Federer could reach only the quarterfinals in the singles draw, bowing out to then No. 8 James Blake, thus ceding his No. 1 ranking to Nadal after being at the top for a record 237 consecutive weeks. Federer captured his only Grand Slam of 2008 at the US Open, defeating Andy Murray in the final.

At the end of the year, Federer suffered a back injury, which caused him to withdraw from the Paris Masters and resulted in a poor showing at the year-end championship, where he was eliminated in the round-robin stage. This marked the only time in his career that he was eliminated before the semifinals. Federer ended the year ranked No. 2.