Like other tournaments, tennis at the Olympics follows seeding protocol. Of the 64 players entered into the draw, 16 of the top-ranked competitors are seeded in both the men's and women's fields.
Then the unseeded players are placed randomly throughout the draw. The main reason for the seeding is to prevent the top-ranked players from meeting in the tournament during the early rounds.
Seeding ensures the later matches will remain competitive, assuming the lower-ranked players are eliminated, leaving the top-ranked players to meet each other in the final rounds.
It would not be good planning if the the top 10 players faced each other in the first round, for example.
Regardless of the "plan," however, sometimes the seeding goes awry because an unseeded player wins over someone ranked higher. This is an upset. This also makes tournaments unpredictable and exciting.
Already in 2012, the No. 6 seed Tomas Berdych was upset by Belgian Steve Darcis in the first round. Following that upset on day one, Agnieszka Radwanska found herself down and out after German Julia Goerges defeated the No. 2 seed, also in the opening round of action.
Like other tennis events, Olympic competition has seen its fair share of upsets since 1988, when the sport returned to the Olympics held in Seoul.
Following are the 25 biggest upsets at previous Olympics since tennis re-emerged as a part of the Summer Games.