WASHINGTON, March 25 (Xinhua) -- Australian qualifier Thanasi Kokkinakis produced the biggest upset thus far in 2018, stunning defending champion Roger Federer at the Miami Open on Saturday.

The 21-year-old Aussie came from one set down to defeat the three-time Miami champion 3-6, 6-3, 7-6(4) in the second round of the elite event.

"It's an unreal week for me. I've trained with Federer a bunch of times and he's a great role model for the sport, but I took what I learned and played my game and executed. I just needed to play my game and aggressive tennis," said Kokkinakis, ranked 175th in the world.

It's the first time the Swiss has dropped consecutive matches since 2014. He was coming off another defeat in a deciding tiebreak, having fallen to Juan Martin del Potro in the BNP Paribas Open final on Sunday.

Moreover, Federer will not retain the No. 1 position in the ATP rankings following the tournament, with Rafael Nadal of Spain returning to the top spot.

"It's disappointing," Federer said. "Don't know why I could never get to any level. Sometimes you have these matches. Sometimes you find a way through. I just couldn't get it done today."