Roger Federer lost his second consecutive match and the No. 1 ranking.
Australian Thanasi Kokkinakis, a qualifier ranked 175th, rallied to upset Federer 3-6 6-3 7-6 (7-4) in the second round at the Miami Open.
The 36-year-old Federer, the oldest man to be ranked world No.1, will lose that spot to Rafael Nadal when the new rankings are released on April 2.
"I deserve it after this match," Federer said.
"That's how I feel."
Kokkinakis became the lowest-ranked man to beat a No.1 since No.178 Francisco Clavet upset Lleyton Hewitt in 2003. That match was also at Key Biscayne.
"Pretty crazy," Kokkinakis said. "I'm pretty happy about it."
Federer has lost back-to-back matches for the first time since 2014, a dip that comes after a career-best 17-0 start to the year.
He lost to Juan Martin del Potro in the Indian Wells final, a defeat that also came down to a winner-take-all tiebreaker.
Did the losses have anything in common?
"Yes, 7-6 in the third," Federer said.
"Other than that, not much."
Kokkinakis, 21, has long been regarded as a promising talent with a thunderous serve and forehand, but has been plagued by injuries.
The match was his first against Federer, although they've practised together.
"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," Kokkinakis said.
Federer won't be playing to reclaim the No.1 spot anytime soon. He said he'll skip the clay season for the second year in a row, including the French Open.
The match in Miami turned when Federer played a poor service game and was broken at love to fall behind 3-1 in the second set.
"That game I knuckled down and put pressure on him," Kokkinakis said.
"I started to dictate from there."
On match point, Federer buried a backhand return in the bottom of the net.
"I've always liked his game," Federer said.
"I'm happy for him that on the big stage he was able to show it. It's a big result for him in his career, and I hope it's going to launch him."
The tournament is just the fifth for Kokkinakis in seven months, but he said he thought he could beat Federer.
"When I'm playing on my terms, I don't feel there are too many people that can go with me," he said.
"I just needed to play my game and aggressive tennis."
Kokkinakis will face veteran Fernando Verdasco in the next round after the 31st-seeded Spaniard defeated countryman Guillermo Garcia-Lopez 6-4 0-6 6-2.
Australian 17th seed Nick Kyrgios will commence his tournament against Dusan Lajovic in the night match.
Fourth seed Alexander Zverev, eighth seed Jack Sock, 10th seed Tomas Berdych and 35-year-old Spaniard David Ferrer all progressed to the third round.
Other second-round winners were Pablo Carreno Busta, Kevin Anderson, Sam Querrey, Fabio Fognini, Borna Coric, Karen Khachanov, Denis Shapovalov, Steve Johnson and Frances Tiafoe.