Holder Novak Djokovic lost 6-7, 6-3, 4-6 to South African qualifier Kevin Anderson in his opening match at the Sony Ericsson Open on Friday.
World number three Djokovic, drained after winning last week's Pacific Life Open, could not handle the power of his lanky, 6-foot-7 opponent.
"On the last point my shoelace was broken but I'm not trying to find excuses," the Australian Open champion told reporters. "He deserved to win.
"I tried to recover but obviously I was a little bit tired from the start."
World number 122 Anderson, who reached the Las Vegas Open final this month as a qualifier, was in disbelief after his first victory over a top-10 player.
"I really don't know what to say right now," he said. "I really tried to give myself the best shot.
"I kept telling myself I can do this, I've just got to believe in myself, play my game."
Djokovic said he would now take a break from tennis.
"There are a lot of expectations as the third-best player in the world," he said.
"The good thing about this loss is I can get a longer period of vacation."
FINAL SET
Djokovic looked in control of the final set when he went ahead 2-0 before Anderson mounted a charge.
When serving at 2-0, 30-40, Djokovic, who bounces his ball constantly before delivering, received a time violation from the umpire but won the point.
"I didn't focus in that moment," said the Serb. "The time violation and a lot of different things affected my service game loss.
"That's okay. It's all part of the game."
It was however the time Anderson started to muscle Djokovic towards the exit doors as he won 13 points in a row to go ahead 3-2, 40-0.
"Ten points went by and I suddenly realised I had won quite a few points in a row," Anderson said.
Anderson double-faulted on his first match point in the 10th game.
On his second match point, Djokovic netted a forehand on a lengthy and entertaining rally.
"I can compete with the best in the world," said Anderson. "I proved that to myself, whether he was on form or off form."
