Who said we're done?

In 2017, Roger Federer returned to the tennis tour after a layoff of about six months, taken to completely recover from a knee injury and subsequent surgery. He went on to win seven titles in the year, including the Australian Open - which he successfully defended this year - and an eighth Wimbledon title.

Rafael Nadal began his 2017 after a three-month layoff taken to recover from a wrist injury. He went on to win six titles - including a record 10th French Open and the US Open - in a year that saw him reclaim the World No. 1 ranking. The Spaniard had a similar 2013, when he returned from six months off due to a knee injury to win 10 titles, including two Grand Slams.

In December 2016, Petra Kvitova was attacked at her home in the Czech Republic by a knife-wielding intruder, and in attempting to defend herself, the two-time Wimbledon champion suffered serious injuries to her left hand - her playing hand.

After what was effectively a career-saving surgery, Kvitova was expected to be out of action for six months, but returned a month earlier than planned at the French Open, where she won one match. In June 2017, she won the Aegon Classic in Birmingham for her first title since the attack.

In 2011, Yuvraj Singh was at the peak of his cricketing powers, having helped India win a second World Cup title. A few days later, after being diagnosed with a cancerous, six-inch tumour in his chest, he flew to the US for treatment.

Not only did Yuvraj make a full recovery and write a book about his struggles, he also made a return to the Indian team in December 2012. The first few years back were hard, though, as he went in and out of the team. Yuvraj then returned to the grind of domestic cricket and earned his way back into the Indian team last year, smashing a career-high ODI score of 150 in his second match back.

In 2017, Yuvraj was one of four Indians in ESPN's World Fame 100 list.

After winning eight gold medals at the 2008 Beijing Olympics, Michael Phelps' already-strained relationship with his longtime coach Bob Bowman began to worsen. By the time the 2012 Games came around, both athlete and coach wanted to quit, but couldn't, and somehow Phelps still won six medals in London. After that, Phelps was done.

A year later, Phelps decided he wanted to come out of retirement, which he eventually did, retiring a second time after the 2016 Rio Olympics as the most decorated Olympian of all time, with 28 medals to his name. Before he returned to competition, though, Phelps had to confront a host of issues, including his struggles with depression and addiction.

In 1990, Monica Seles became the youngest ever French Open champion at 16. Before she turned 20, Seles had already won eight Grand Slam singles titles and become World No. 1, in addition to having a healthy rivalry with Steffi Graf. She had what many considered the potential to become the most accomplished female player ever.

On April 30, 1993, during a match at a tournament in Hamburg, Germany, Gunter Parche, an obsessed fan of Graf, ran to the edge of the court during a break between games and stabbed Seles with a knife between her shoulder blades.

Although her injuries healed soon, Seles didn't return to competitive tennis for more than two years. After her return, she won her fourth Australian Open, was runner-up at three more majors and won bronze at the Sydney Olympics, but her career was never quite the same after the stabbing. She played her last professional match in 2003, but officially retired only in 2008.

Sandeep Singh made his international debut for the Indian hockey team in 2004 in the Sultan Azlan Shah Cup. In August 2006, while travelling by train to join the national team due to leave for the World Cup, he was hit by an accidental gunshot, leaving him bound to a wheelchair for the next two years of his life.

Not only did he recover from that, Sandeep went on to reestablish his place in the side, leading India to a Sultan Azlan Shah Cup victory in 2009, and was also awarded the Arjuna Award in 2010 for his efforts in the sport.

Evonne Goolagong Cawley, Kim Clijsters and MC Mary Kom have not just returned to sport, but done so successfully after becoming mothers.

Australia's Goolagong, winner of seven Grand Slam singles titles, returned to the tour after giving birth in 1977, going on to win Wimbledon in 1980.

After retiring in 2007, partly due to injuries, Belgium's Clijsters planned to return to the sport in 2009, a year after the birth of her daughter. Clijsters was granted a wildcard to the US Open, which she went on to win and successfully defended the next year for her second and third Grand Slam titles. Clijsters also won the Australian Open in 2011, going on to retire again after the 2012 US Open.

After her marriage in 2005 and the birth of her first two children, India's Mary returned to boxing and had considerable success, including an Asian Games gold and an Olympic bronze. After gold at the Asian Women's Championships and at the India Open most recently, Mary has her sights set on this year's Commonwealth Games.