By Caroline Cheese
BBC Sport at Wimbledon
Former champion Venus Williams crashed out 7-6 (7-5) 7-6 (8-6) in the second round to rising star Karolina Sprem.
However, the match was overshadowed by controversy at 2-1 in the second-set tiebreak when the umpire incorrectly awarded a point to Sprem.
Neither player picked up on the error and Williams went on to earn three set points but faltered dramatically.
Sprem, who had served for the match at 5-3, finally wrapped up a stunning win when Williams went long with a forehand.
A dejected Williams refused to blame the confusion in the tiebreak for her defeat.
"I don't think one call makes a match," said the former world number one.
"It was very confusing but sometimes I lose track of the score and at the time I just felt maybe that was what had happened so I just carried on."
Tournament referee Alan Mills said the result would stand despite the umpire's error.
"It is a bizarre situation but ultimately the result stands and I'm afraid Venus has left the tournament," Mills told BBC Sport.
"I would have thought the linespeople would have thought they could go up to the umpire if they realised a mistake had been made.
"What I cannot understand is the players' reaction. It is the players' responsibility once they are on court to know the score."
Sprem said she did not notice the error.
"I was confused but I didn't think about it," she said. "I was 100% in the match and I just wanted to get on and play."
Sprem said her victory made up for her defeat to Williams in Berlin in May, when she lost despite leading 3-0 in the deciding set.
The 19-year-old, world-ranked 30th, matched Williams from the baseline and clinched the first set when the American made three careless errors in the tiebreak.
The Croat seemed to have missed her chance when, two points from victory in the second set, the nerves struck and she surrendered her serve at 5-3.
The set went to another tense tiebreak.
At 2-1 in Williams' favour, Sprem's first serve went wide but the rally continued and she ended it with what would have been a winner.
But the umpire missed the linesman's initial call and gave the point to Sprem, even though the Croat then delivered a second serve from the same side.
Both players looked confused but no one pointed out the error and Williams powered to a 6-3 lead.
However, the third seed allowed Sprem to level at 6-6 and Williams could not recover.
"I did my best. I just think she played really well and deserved the win," said Williams.
"I would love to have done better but it's impossible to do it every year.
"This was not my event, but I'll be ready for the next one. I will figure what I did wrong and try not make the same mistake twice."