Dominic Thiem said he was "in the zone" during his thrilling ATP Finals win over Novak Djokovic on Tuesday.
The Austrian booked his spot in the semi-finals in London by overcoming Djokovic 6-7 (5-7) 6-3 7-6 (7-5) at the O2 Arena.
Thiem, who beat Roger Federer in Group Bjorn Borg on Sunday, said it was an unforgettable performance.
MORE: Andy Murray would back himself against Nadal, Djokovic or Federer
"I was in the zone from the first point on," he said in an on-court interview.
"I served for the match at 6-5 in the third set, but obviously I was playing the best returner in the game so I didn't worry too much, I was focusing on the tie-break and coming back from 4-1, a little bit of luck here and there.
"But, in general, it was just unbelievable and a match that I will probably never forget."
Thiem hit 51 winners during his win, coming from 4-1 down in the third-set tie-break to edge past Djokovic.
London, you’ve just witnessed a CLASSIC.@ThiemDomi defeats Djokovic 6-7(5), 6-3, 7-6(5) to book his spot in the #NittoATPFinals final four
— ATP Tour (@atptour) November 12, 2019
: @TennisTV pic.twitter.com/34arwXNxRG
The 26-year-old was delighted with his performance and said it was the level required to beat the 16-time grand slam champion.
"This was really one of these very special matches that I practised all my life for, what I practised all my childhood for, a really epic one in front of an amazing atmosphere, beating a real legend of our game," Thiem said.
"I couldn't be happier and also I qualified for the semi-finals which is the best."
He added: "I was playing Novak, who is in great shape, who is probably the best player in the world right now.
"I had to do something special and luckily a lot of these balls and these winners went into the court."
Thiem will face Italian Matteo Berrettini in his final match in the group on Thursday.