Novak Djokovic was gracious in defeat and expressed his pride over "a most fantastic last five months" after Karen Khachanov halted his 22-match winning streak to claim Paris Masters glory in a stunning upset on Sunday.
The 14-time grand slam champion has had a remarkable 2018 till now.
Although Djokovic broke in the fourth game to move 3-1 up and then led 30-0 on serve, the unseeded Khachanov broke him straight back and the momentum abruptly shifted away from Djokovic.
Djokovic also defeated Federer in the 2013 Rolex Paris Masters semi-finals.
Khachanov soon snatched the break back and, after edging ahead at 6-5 with another break, sealed the set in 58 minutes with his third ace.
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Jurgen Klopp's side took the lead through James Milner in the 61st minute but the Gunners hit back through Alexandre Lacazette . An open first stanza had Klopp cutting a frustrated figure on the touchline, unhappy with the team's defensive shape.
Djokovic will face Khachanov for only the second time in tomorrow's final after the 22-year-old beat Thiem 6-4, 6-1.
"Karen played really well and he deserved to win", Djokovic said. 'I couldn't be happier to finish the season like this'.
The third break arrived for Khachanov in the third game of the second set. "I'm really pleased to overcome this kind of challenge".
It is a first Masters title for Khachanov, who has now demanded he be regarded a contender for the 2019 Australian Open.
Novak Djokovic celebrates after winning his semi final match against Switzerland's Roger Federer.
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Instead, up against a great champion looking for a fourth consecutive tournament win, following his victories at the Cincinnati Masters, the U.S. Open and the Shanghai Masters, the chess-loving Russian changed his strategy.
He said: "I am very happy to come to the finals but I didn't feel too good today".
Thiem joined Djokovic and Federer in the Nitto ATP Finals field when Kei Nishikori lost on Friday.
Khachanov became the third different first-time victor of a Masters tournament this season - joining Juan Martin del Potro and John Isner - and will rise to a career-high ranking of 11th on Monday.
Four-time Paris champion Djokovic made the last-eight when Bosnian opponent Damir Dzumhur retired injured trailing 6-1, 2-1.
"I would like to give a lot of respect to Karen for playing such fantastic match today and he deserves to win".
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