The mystifying Serena Williams moment

Editor's note: For two weeks, starting Dec. 8, ESPN.com is unveiling its 2014 tennis awards once per day.

Dec. 8: Men's player of year | Dec. 9: Women's player of year | Dec. 10: Top men's matches | Dec. 11: Top women's matches | Dec. 12: Top shot-makers | Dec. 15: Top shots | Dec. 16: Top tirades | Dec. 17: Most mystifying moments | Dec. 18: Top on-court moments

It was a season full of surprises on the tour, with plenty of twists, turns and talk. But some unfolded in particularly unusual fashion, arresting attention as they happened. Here are some of the moments that left tennis watchers gaping.

1. Serena Williams' Wimbledon exit

She had already been ousted from the singles in a dramatic three-setter, but Serena would make an even more eventful exit in doubles. With her sister Venus sitting expressionless beside her, Serena called for the trainer during the warm-up, saying her vision and balance were affected. She then attempted to play the match, sending the ball flying when making contact in the first game. Next Williams hit four double faults that often weren't even in the vicinity of the service box, retired from the contest and, ultimately, left the tournament before giving a news conference. And she has still not given a more specific explanation than illness, saying she planned to have tests in the offseason.

2. Li Na’s retirement

Li won her second Grand Slam at the Australian Open, injured her knee and had a sharp fall in her results during the clay season, had valued coach Carlos Rodriguez leave her following Wimbledon and withdrew from the US Open. With retirement rumors swirling, Li announced she was stopping just before her hometown tournament, which was to be played for the first time this year with her as its top attraction. Quite a roller-coaster season.

3. Rafael Nadal's injury issues

Nadal's physical problems produced some unusual sights on the court this season. He began experiencing back problems in the warm-up during the Australian Open final against Stan Wawrinka and called the trainer down a set and 2-0. Nadal had to leave the court to be examined, which led to a break of about 10 minutes, causing Wawrinka to complain angrily to the umpire. Nadal was even booed when he returned to court, but it was apparent the Spaniard was really hurt when he began serving at about 80 mph and could barely stay in points. He then improved and somehow won the third set against a nervous Wawrinka, but the first-time Grand Slam finalist was able to clinch the match in the fourth -- one of the stranger Grand Slam finals ever played.

Nadal had then only just returned from a wrist injury when he developed appendicitis in Shanghai, but chose to take antibiotics and keep playing rather than have surgery right away. Initially unable to practice and not sleeping properly because of the painful condition, he had poor showings in Shanghai and Basel and then decided to stop for the season.

4. Miami's men's semifinals

The Miami semifinals were supposed to feature Nadal versus Tomas Berdych and Djokovic versus Kei Nishikori, but instead saw no action when Nishikori withdrew with a hip injury and Berdych followed suit with a stomach problem. It wasn't the only no-show day on the men's circuit in 2014. Roger Federer withdrew from the final of the ATP World Tour Finals against Novak Djokovic because of a back injury.

5. Eugenie Bouchard's Montreal appearance

It was one of the most hyped appearances of the year -- Montrealer Eugenie Bouchard playing her hometown event in her first tournament since reaching the Wimbledon final. There were record ticket sales, television cameras and packed crowds as she walked on court for her opening round against little-known qualifier Shelby Rogers. Clearly affected by the attention, Bouchard didn't win a game in the first set and told her coach, Nick Saviano, that she wasn't even in the match. She gathered herself and took the second set, but then unraveled -- the 6-0, 2-6, 6-0 score line telling the story of a very ragged match.

6. Stan Wawrinka's Davis Cup faceoff

The fifth match of the Davis Cup final wasn't played as Switzerland took a winning 3-1 lead against France, but Wawrinka still had a faceoff with the French team that evening. A slightly tipsy Wawrinka had said during the news conference that the French players had "talked too much" going into the tie, and they confronted him in the bathroom about his comments during the Davis Cup dinner. French player Julien Benneteau told a French newspaper that Wawrinka apologized and there was no violence, but a heated five-minute discussion ensured before Gael Monfils stepped in to calm things down.

7. Shuai Peng's US Open collapse

A hotly contested match on a hot day at Flushing Meadows was too much for Shuai Peng, who collapsed on court during the second set against Caroline Wozniacki. The trainers conducted a lengthy examination that determined Peng was experiencing heat illness, not cramps, which allowed her to receive medical attention midgame. Though advised not to keep playing, she resumed for five more points before again falling to the court and retiring. The scenes of Peng on the ground crying and being wheeled off court led to tournament officials being criticized both for giving her too much assistance, disrupting the match, and for not giving her enough, potentially compromising her health.

8. Andy Murray's referendum tweet

Having insisted that he did not plan to publicly express an opinion on this year's referendum on Scottish independence, Andy Murray caused waves by sending a tweet that morning apparently supporting the “yes” side.

It became one of the most retweeted comments of the referendum, prompting so much negative reaction from the opposing side there was even a police investigation into some of the comments. Murray subsequently said it had been an impromptu decision, and that while he did not regret expressing an opinion, he would now have done it a different way.

9. Spain's Davis Cup captaincy

Former WTA player Gala Leon Garcia, now sporting director of the Spanish tennis federation, was appointed to select Spain's next Davis Cup captain when Carlos Moya decided to leave his position because he was unhappy that the top players had declined to show up for the World Group playoffs. Garcia chose herself, becoming the country's first female Davis Cup captain and setting off controversy. Prominent players like Nadal, Fernando Verdasco and Feliciano Lopez, as well as Nadal's coach and uncle, Toni, complained they had not been consulted and criticized her lack of coaching experience, especially on the men's tour. That led to accusations of sexism, particularly on the locker-room question, which Nadal countered by saying the new captain was misrepresenting her critics' position. Next season Spain will attempt to rebound from relegation and get the top players playing again.

10. Simona Halep's coaching changes

Simona Halep had her best season yet, reaching as high as No. 2 and making a Grand Slam final at the French Open. Much of it was while working with Wim Fissette, the former coach of Kim Clijsters, but Halep decided to get rid of him as the offseason began. There were plenty of questions about why she would change a winning formula, but then again, maybe she wasn't. Halep had done exactly the same thing a year ago, dropping Adrian Marcu despite getting to No. 11 and winning a host of titles. Perhaps her next coach shouldn't do such a good job, if he wants to keep his job.