Serena, who won the title at Wimbledon last year, was stunned by a subliminal performance by French star Marion Bartoli who battled through to a straight sets victory 6-3 7-6 (8-6). She will now play Sabine Lisicki in the quarter-finals.
To make matters worse for Serena - she spent almost a year out with injury
- she has fallen to a world ranking low of 180th, reports the BBC
Bartoli was outstanding in the Wimbledon 2007 semi-final against Justine Henin where she eventually lost in the final to Venus Williams. Bartoli said after the game: "I'm extremely happy about what I did today, especially against Serena, who's extremely tough to beat, It was great performance but tomorrow is another day and I can't think too much ahead."
Venus meanwhile was beaten by Tsvetana Kirilova Pironkova, a Bulgarian rising star who is only ranked number 33 in the ATP rankings
. Pironkova beat Venus Williams 6-2 6-3 in a match played in searing south London heat of around 34c (93f)
Earlier in the Monday play, World number one Caroline Wozniacki crashed out as Dominika Cibulkova overcame the 20-year-old in three sets to make it through to the quarter-finals. On Friday Serena Williams had complained about being moved to Court Two which holds holds 4,063 spectators
– 7,330 fewer than Court One and 10,916 fewer than Centre Court.
There is just a week left in the Wimbledon
finals as the competition reaches the quarter-final stages now.