"I knew I was playing pretty good today".
Djokovic was chasing his fifth Rogers Cup title but was beaten in three sets by Tsitsipas, who turns 20 this weekend.
"I was waiting and I grabbed him like a bulldog and I stuck there", Tsitsipas said.
A win for Djokovic would set up a potential quarter-final clash with German second seed Alexander Zverev, who advanced to the third round after a 6-4 6-4 win over Bradley Klahn.
"This was the best match of my career", Tsitsipas said. "I just played not that great".
"It was a good match, a very positive victory for me over a tough opponent", Nadal said.
The Greek teen made some errors near the end of the second set that allowed Djokovic, a four-time victor of the tournament, to set up a decider.
"I actually feel like there is more time now than before because the shot clock starts counting down once the chair umpire calls the score", Djokovic said.
"I didn't return well".
He next faces Wimbledon runner-up and fourth seed Kevin Anderson who defeated Ilya Ivashka of Belarus 7-5, 6-3 to also make the last-eight.
Top seed Rafa Nadal battled past Swiss wild card Stan Wawrinka 7-5 7-6 (4) and will next meet Marin Cilic, who took down Argentinian Diego Schwartzman 6-3 6-2.
Chelsea striker Batshuayi joins Valencia on loan
The report also stated that Atletico Madrid and Valencia are also interested in signing Michy Batshuayi from Chelsea this summer. But the deal may raise eyebrows from some Chelsea fans, given the forward options left at the club after Batshuayi's departure.
He will face Robin Haase of the Netherlands in the third round on Thursday.
He took advantage of a Djokovic unforced error for an early break at 4-2 and used a few 210-km/h serves to take the first set by holding at love.
He held serve to open the set and wrong-footed Djokovic after the veteran served at deuce, setting up an overhead smash for the advantage before a brilliant backhand victor secured a 2-0 lead.
TORONTO-Stefanos Tsitsipas has been looking virtually unstoppable in recent weeks.
"I was really not focusing on winning or losing, just on starting to play good tennis and start building the right habits".
"Honestly I'm not sure what he said. It gave me the win at the end".
The 19-year-old forced the former world number one Djokovic onto the back foot in the first set but was unable to wrap up a straight-sets win as the 13-time Grand Slam champion claimed the tiebreak with the Greek firing long having saved two set points.
They beat French duo Pierre-Hugues Herbert and Nicolas Mahut 4-6, 6-4, 10-7.
Rafael Nadal was made to wait before his second-round Rogers Cup match due to several rain delays but did his part to put the schedule back on track with a speedy 6-2 6-3 win over Frenchman Benoit Paire on Wednesday.
The number 27 will play the first Masters 1000 quarter-final of his career.
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