10 biggest moments of 2015, plus five questions ahead of 2016 season

The 2015 ATP and WTA seasons concluded at the year-end championships in London and Singapore and 2015 will soon come to a close as well. But as the players gear up for the first major of the 2016 season in Melbourne, Australia, let's take one last look back at an unforgettable 2015 tennis season with the 10 biggest moments of the year, including record-breaking wins, major upsets, renewed rivalries and much more. Based on the 2015 season, we also look ahead with five burning questions ahead of the 2016 season.

Serena topped Sharapova in the final in Melbourne in January to win her sixth Australian Open title and 19th major overall. She captured the title with an ace—twice. Serena fired what appeared to be an ace, but the umpire claimed to have heard a let. Serena stepped to the line, fired the same serve—this time without a let—to seal the win.

Serena Williams announced her return to Indian Wells to end her 14-year boycott of the tournament. She told Sports Illustrated's S.L. Price that returning to Indian Wells this year was the best moment of her tennis career.

By winning Indian Wells (d. Federer) and Miami (d. Murray), Djokovic became the first man to complete the double three times in the Open Era. 

No. 8 Wawrinka defeated top-seed Djokovic 4–6, 6–4, 6–3, 6–4 to win his first title at Roland Garros and second major overall. The Swiss upended Djokovic's quest for his first French Open title, the only major that has eluded him in his career. 

​Djokovic defeated Federer 7-6 (1), 6-7 (10), 6–4, 6–3 in an epic Wimbledon final to win his third title at the All England Club and ninth major title overall. In a rematch of last year's final, Djokovic outclassed Federer with his consistent all-court game to become the first man to defend a Wimbledon title since Federer in 2007. 

Italian journeywoman Roberta Vinci shocks Serena Williams in the U.S. Open semifinals, ending her quest for the calendar year Grand Slam with a 2–6, 6–4, 6–4 win over the World No. 1. 

​Nadal had won 151-straight matches when leading two sets to love in a major match, but Fognini rallied in the third round of the U.S. Open to upset Nadal and end his run. 

After the first all-Italian U.S. Open women’s final between Roberta Vinci and Flavia Pennetta, Pennetta capped off her win and first major title of her career by announcing that she will retire at the conclusion of the 2015 season during the trophy ceremony on Arthur Ashe.

After a 1–2 record in the round robin matches, Radwanska beat Petra Kvitova in the final to win the biggest title of her career in Singapore. 

Murray defeated Goffin in straight sets in the decisive rubber to lift Great Britain over Belgium and win the title. 

All eyes will once again be on the World No. 1 once she returns to the Tour in 2016 for her first WTA match since her loss at the U.S. Open. Can she bounce back after her disappointing result in New York? Many believe that she won’t be capable of matching her spectacular 2015 season, but Serena seems as motivated as ever heading into Australia. Her health will play a big role in her success this season.

Can Novak Djokovic complete the Grand Slam in 2016?

Three of the five major events in 2016—the Rio Olympics will be held in August—will be on hard courts, Djokovic’s beloved surface. With 10 majors under his belt after the 2015 season, the Serb seems primed to match Nadal’s mark of 14 Grand Slam titles. But history isn’t on Djokovic’s side: The past three times a man has won three of the four Grand Slams—Federer in 2007, Nadal in 2010 and Djokovic in 2011—he reached three major finals the following year, but only captured one title.

Nadal struggled with consistency and nagging injuries but still finished the 2015 season ranked the No. 5 on the ATP Tour. Will 2016 be a rebound season where Nadal claims his 10th French Open title or will the 14-time Grand Slam champion once again go without winning an ATP Masters or major title?

Can Venus Williams win another Grand Slam?

Venus finished in the Top 10 and reached the quarterfinals at a Grand Slam (twice) for the first time in five seasons in 2015. The No. 7-ranked Williams sister will turn 36 in June 2016, but she seems to be peaking after battling health issues for the past few years. If Venus can continue to beat out young challengers such as Belinda Bencic, Madison Keys and Karolina Pliskova, she has a shot at maintaining her Top 10 ranking and making a deep run at a major in 2016.

After a quarterfinal showing at the French Open and a runner-up trophy at Wimbledon, 22-year-old Muguruza surged from No. 21 to No. 3 by the end of the 2015 season. The Spaniard has a chance to break through in 2016 but she’ll be up against the weight of expectation, something we’ve seen Eugenie Bouchard and other fresh-faced Slam finalists struggle with following a breakout season.