It usually pays to be a top seed in tennis. Early upsets, while they happen, arent commonplace. But at the Rio Olympics, being a favorite has become a curse.Through four days, the Games have lost a host of star players, making this one of the more bizarre, unpredictable events in recent memory.Heres a recap of the first four days:Day 1Kirsten Flipkens def. No. 5 Venus Williams 4-6, 6-3, 7-6 (5)Few athletes have worn the red, white and blue prouder than Venus, a four-time Olympic gold medalist (1 singles, 3 doubles). However, Venus, the fifth seed, lost a heartbreaking 4-6, 6-3, 7-6 (5) opening-day match in Rio to Flipkens, a Belgian ranked No. 62 in the world. Venus did not speak to reporters afterward, but U.S. coach Mary Joe Fernandez told the media Venus has been feeling ill before leaving for the Olympics. The same day, No. 4 Agnieszka Radwanska suffered a straight-sets loss against Saisai Zheng.Day 2Juan Martin del Potro?def. No. 1 Novak Djokovic 7-6 (4), 7-6 (2)The incredible shot-making and raw emotion will be remembered as much as the magnitude of this loss by Djokovic. Although Del Potro, the 2009 US Open champion, has fought back from multiple wrist injuries, the reality was that many believed he would never return to the player he once was. But the unseeded Argentine crushed serve after serve and forehand after forehand from the start. Djokovic never had a break-point opportunity in the match. Afterward, del Potro broke down in tears after his biggest win since that US Open seven years ago, while Djokovic was equally as distraught after failing in his quest to capture a gold medal that has eluded him throughout his career.Day 2Lucie Safarova/Barbora Strycova?def. No. 1?Venus Williams?and Serena Williams 6-3, 6-4Riding a 15-0 record heading into Rio, never before had the Williams sisters lost an Olympic doubles match together. But with Serena playing her second match of the day, and Venus still suffering from health issues that had plagued her the day before, the top seeds fell in straight sets to a Czech team that wasnt supposed to be playing together in the first place. (Strycova was a late replacement for Karolina Pliskova.) On the mens side, the No. 2-seeded Murray brothers, Andy and Jamie, also lost.Day 3Gilles Muller def. No. 5 Jo-Wilfried Tsonga 6-4, 6-3The third day of Rios tennis event was the only one without a monumental upset. However, Frances top player, Tsonga, fell rather meekly to hard-serving countryman Muller. Tsonga had entered Rio riding a wave of momentum after reaching the Wimbledon quarterfinals.Day 4No. 15 Elina Svitolina def. No. 1 Serena Williams 6-4, 6-3Serena has had these kind of days before, as evidenced by last years US Open semifinals. But what we saw on Day 4 in Rio was a tentative, sloppy player whose greatest weapon -- her serve -- went haywire. 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If their patchy late-season results continue this week against Collingwood, they could drop as low as seventh on the ladder and relinquish the double chance.As for the Kangaroos, they remain 10th in the Power Rankings after a third-straight loss; North fans may only be buoyed by the fact thheir spot in September is secure after Melbourne capitulated against Carlton.dddddddddddd. ?Unfortunately for both the Dees and the Saints, they remain above the Kangaroos on the ESPN table but are certain to miss finals despite being in much better late-season form.LegendForm: The higher the number, the better a teams recent form.Draw: The higher the number, the easier a teams upcoming draw.Fitness: Determined by the number and quality of fit players available. A list in good shape is ranked around a 7; a list missing three or four best-22 players might be considered a 3 or 4.X-Factor: The number and quality of genuine match-winning players.Attack: Potency of the teams offence.Defence: Ability of the team to prevent the opposition from scoring. ' ' '