WIMBLEDON, England - The serve was clearly out. The line judge called it out. Both players knew it was out.

Everybody on Centre Court seemed to know it was out - everybody, that is, except for the chair umpire.

The scoring error by umpire Ted Watts contributed to Venus Williams' stunning upset at Wimbledon, a result that erased the chance of a third-straight All England Club final against sister Serena.

''I like to think he didn't do it on purpose,'' the two-time champion said after losing 7-6 (5), 7-6 (6) to Croatia's Karolina Sprem in a second-round match Thursday.

The mistake by Watts - which gave Sprem an extra point during the second tiebreaker - didn't decide the match. But it certainly could have influenced the outcome, and it left players, officials and fans wondering how such a thing could ever happen.

The error came with Williams leading 2-1 in the tiebreaker. Sprem's first serve to the ad court was wide, and the line judge put her arm out and shouted, ''Fault!''

Williams casually hit the ball back. Sprem hit a backhand into the open court with Williams standing still, figuring the point was over.

Watts incorrectly called the score as 2-2, giving Sprem a point when instead she should have been taking a second serve.

The players took no notice and lined up in the same positions for what they thought was a second serve. Sprem hit an 86 mph serve, and Williams responded with a backhand return winner.

That should have made the score 3-1, but Watts called it 3-2.

Both players appeared confused, but neither disputed the call.