Cape Town - Novak Djokovic believes Nick Kyrgios' powerful serve was his biggest weapon as he was knocked out of the BNP Paribas Open on Wednesday.
The former world number one has now played four sets of tennis against the Australian and lost them all.
This loss at Indian Wells marks the second consecutive tournament where Kyrgios has knocked him out.
In an interview afterwards, Djokovic - who was once hailed as the best returner in the game - heaped praise on Kyrgios’ accurate and unreadable serve.
"On his first serves, to try to anticipate and read his serve, where
he's going to go 140 miles per hour down the T and also pretty good
angle wide, it's hard to position yourself well... it is a gamble," the
12-time grand slam winner told the ATP World Tour website.
"His second serve, if you think you're going to have a look at it, you don't, because he goes for it as well. He didn't make too many double faults."
Kyrgios' win ends Djokovic's impressive 19-match winning streak at Indian Wells, where he won three years in a row from 2014.
Despite Wednesday's disappointing loss, Djokovic was proud of his amazing run in California.
"The run was amazing... I am very proud of it, obviously," he added.
"It had to end at some stage. Unfortunately, it was today. Nick, again, as he did in Acapulco earlier, a few weeks ago, he served so well.
"Just wasn't managing to get a lot of balls back on his serve, first and second, as well. That's what made a difference. He obviously comes out playing his style, very aggressive, and just going for every serve, whether it's first or second. It's obviously very hard to play like that."
Kyrgios faces Australian Open champion Roger Federer in the quarter-finals on Friday.