Michael Phelps won a 22nd Olympic gold but doping reared its ugly head again Friday as Chinese swimmer Chen Xinyi and Bulgarian athlete Silvia Danekova became the first major cases of the Rio Games.

Chen's failed test after narrowly missing a bronze medal in the women's 100-metre butterfly was announced as athletics headed into the Olympic spotlight needing an antidote to its own doping scandals.

Egyptian judoka Islam El Shehaby was jeered out of the Olympic arena in Rio on Friday after refusing to shake hands with his Israeli opponent.

Jose Mourinho insisted Friday that Manchester United will be contenders for the Premier League title in his first season in charge at Old Trafford, and criticized some of his rival managers for not being brave enough to make the same claim.

Ethiopia's Almaz Ayana smashed one of the longest-held records in athletics here Friday to win the 10,000m and claim the first gold of the Olympic track and field competition.

The 24-year-old pulled away from the field midway through the race and never let up before powering home in 29min 17.45sec.

Turkey issued an arrest warrant for its former star footballer Hakan Sukur over last month's failed coup as Ankara said Friday it had detected the first progress in persuading the United States to extradite the alleged mastermind of the putsch.

With the shockwaves from the July 15 coup aimed at unseating President Recep Tayyip Erdogan shaking Turkey four weeks later, Foreign Minister Mevlut Cavusoglu said 32 diplomats recalled to Ankara were still missing.

Golf prepared to make its controversial return to the Olympics on Thursday after another dramatic day in the pool which saw Australia's new teenage sensation Kyle Chalmers win the blue riband 100m freestyle gold.

Brazilian professional Adilson da Silva was due to hit the first Olympic golf shot for 112 years at 7:30 am (1030 GMT) on a Rio course that has a caiman crocodile or two hiding among the traps.

Police in London are investigating allegations that a young tennis player was poisoned while competing at this year's Wimbledon, a British newspaper reported on Thursday.

Rafael Nadal reached the Olympic Games quarter-finals Thursday, winning the first of 10 matches he will need to sweep in just four days if he is to secure triple Rio gold.

The 2008 champion defeated Gilles Simon 7-6 (7/5), 6-3 for his eighth win over the Frenchman in nine meetings.

A Namibian boxer arrested for alleged sexual assault at the Rio Olympics athletes village will fight as scheduled on Thursday, officials said.

Jonas Junias Jonas, 22, was detained on Sunday for allegedly sexually assaulting a female room cleaner, police say. It was the second sex assault case to rock the Games village.

Top Brazilian athletes Rafaela Silva and Joanna Maranhao have hit back at racist and sexist insults that have caused Olympic heartbreak and tears.

Rio gold medal judoka Silva, who was called a "monkey" after crashing out of the London Games four years ago, and swimmer Joanna Maranhao, who has had rape threats in recent days, say they will use the hatred to fuel change.