RIO DE JANEIRO – This is goodbye for Michael Phelps. The most decorated athlete in Olympic history has one more race before retirement.

His shot at a 28th medal comes Saturday in the final slate of swimming at the Rio de Janeiro Games, where Usain Bolt thrilled the crowd at Olympic Stadium in his first appearance of the games and Rafael Nadal’s bid for an unprecedented second Olympic singles gold medal ended with a thrill-a-minute semifinal loss to Juan Martin del Potro.

Monica Puig won Puerto Rico’s first gold medal in any sport in Olympic history, upsetting Angelique Kerber in the women’s tennis singles final at the Rio Games. Puig is ranked 34th and is the first unseeded women’s singles gold medalist since tennis returned to the Olympics in 1988.

Seeded second, Kerber won the Australian Open in January and was the runner-up at Wimbledon last month. She was trying to give Germany its first singles gold since Steffi Graf in 1988, but instead goes home with silver.

With four gold medals at these games and 22 for his career, Phelps needs some help from his teammates to win another – he leads off with the butterfly leg of the 400 medley relay.

There were 22 golds at stake on a sun-splashed Day 8 of the Rio Games in 13 sports, including fencing, track cycling, boxing, shooting, trampoline and weightlifting.

While competitions are wrapping up in swimming and rowing, they’re just heating up in track and field, which will award five golds: for the women’s 100 meters and the heptathlon, and the men’s 10,000 meters, discus and long jump.

The IAAF said Saturday it has banned the only Russian in Olympic track and field from competition and she is appealing the ruling. A decision on Darya Klishina’s appeal is expected before the long jump competition begins Tuesday.

Bolt turned the Olympic Stadium into his stage again for his first race at the Rio Games, cruising to an easy win in his 100-meter heat, backed up by the trademark showmanship to the thrill of the crowd.

The final five gold medals were up for grabs in the pool.

Phelps was beaten in his last Olympic individual race Friday night by a 21-year-old who grew up idolizing him. Joseph Schooling of Singapore built a big lead in the 100 butterfly and easily held off one of Phelps’ patented rallies.

Phelps insists he won’t be coming out of retirement again for Tokyo in 2020.

“I’m not going four more years and I’m standing by that,” he said. “I’ve been able to do everything I’ve ever put my mind to in the sport and after 24 years in the sport, I’m happy with how things finished.”

Other highlights from Day 8:

GOLF GOLD: For all the talk about top players skipping Rio, the sport couldn’t have asked for a better final round. Justin Rose of Britain is the third-round leader and will be trying to win golf’s first gold medal in 112 years. Rose made two eagles in the opening five holes and shot a 6-under 65 to build a one-shot lead over British Open champion Henrik Stenson.

SLUGGISH SPRINTER: Bolt did not disappoint the crowd, but he was displeased with himself. He was slow out of the blocks but once his tall majestic frame reached full flow, there was no stopping him. “It wasn’t the best start, it felt kind of sluggish,” Bolt said, adding that he never has been a morning person and preparations for his noon race felt unusual.

GOODBYE GREEN: After blaming algae, heat and even the number of swimmers for the green water at Maria Lenk Aquatics Center, officials now say a contractor mistakenly dumped hydrogen peroxide into the pool, causing an adverse reaction with chlorine. So, they’re draining the larger pool to transfer nearly 1 million gallons from a nearby practice pool.

PHOTO FINISH: After rowing for more than a mile, the gold medal came down to an inch. New Zealand’s Mahe Drysdale and Croatia’s Damir Martin crossed the finish line in the men’s single sculls side by side, not knowing which one of them was the new Olympic champion. A photo analysis showed Drysdale won by a thumb’s length.

BRITISH ELATION: Another British medal haul is going on at the velodrome. Britain won seven of 10 gold medals possible in London in 2012. The women’s team pursuit squad of Katie Archibald, Laura Trott, Elinor Barker and Joanna Rowsell-Shand beat the world champion United States in the finals, giving their powerful track cycling team its third gold medal in Rio.