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Greek fifth seed Stefanos Tsitsipas reached his second Grand Slam semifinal as he raised his game after a slow start to defeat Russian Andrey Rublev 7-5, 6-2, 6-3.
While Djokovic, who had lost five of his nine previous French Open semi-finals, was given a fright it always looked as though his vast experience would prove decisive.
World number 11 David Goffin was another loser, along with Italy's Fabio Fognini, ranked one place below him.
Alexander Zverev and Stefanos Tsitsipas both crashed to defeat in their first matches of the year Friday as day one of the inaugural ATP Cup sprung some big upsets.
The world No. 1 took one hour and 32 minutes to defeat the rising Greek star.
Tsitsipas was unable to claim his first major title at this year's Slams, two of which were won by the 33-year-old Nadal and two by the 32-year-old Djokovic, while the 38-year-old Federer came close to adding to his record 20 titles.
The 21-year-old debutant, the youngest of the eight qualifiers for the elite season-ender at the O2 Arena, displayed resilience and flair in equal measure to become the youngest champion since Lleyton Hewitt in 2001.
Earlier, Stefanos Tsitsipas shocked six-time champion Roger Federer 6-3, 6-4 to reach the final of the ATP Finals in his tournament debut.
The Spaniard beat defending champion Stefanos Tsitsipas 6-4, 4-6, 6-2 in the last round-robin match in the London Group.
Results 31-39
