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.

Top 10 moments of 2015

Serena Williams beats Maria Sharapova for the 16th straight time

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 returns to Indian Wells

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

Novak Djokovic completes the triple-double

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.

Stan Wawrinka stuns Novak Djokovic in the French Open final

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.

Novak Djokovic beats Roger Federer in Wimbledon final

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.

Roberta Vinci stuns Serena Williams

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.

Fabio Fognini becomes the first to defeat Rafael Nadal from two sets to love down in a Grand Slam

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.

Flavia Pennetta claims U.S. Open title and makes surprise announcement

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.

Agnieszka Radwanska wins WTA Finals championship

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.

Andy Murray leads Great Britain to first Davis Cup title in 79 years

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

Five questions ahead of the 2016 season

How will Serena fare in 2016 after her U.S. Open loss to Vinci?

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.

Top 100 photos of 2015

Alize Cornet

Hopman Cup

Karolina Pliskova

Brisbane

Madison Keys

Brisbane

Venus Williams

Auckland

Venus Williams

Auckland

Agnieszka Radwanska and Jerzy Janowicz

Hopman Cup

Juan Martin del Potro

Sydney

Roger Federer

Sydney Harbor

Federer

Brisbane

Serena Williams

Australian Open

Bernard Tomic

Australian Open

Kim Sears Murray

Australian Open

Caroline Wozniacki

Australian Open

Nick Kyrgios

Australian Open

Madison Keys, Serena Williams

Australian Open

Maria Sharapova

Australian Open

Novak Djokovic

Australian Open

Serena Williams

Australian Open

Novak Djokovic

Australian Open

CiCi Bellis, Serena Williams

Miami Open

Rafael Nadal

Miami Open

Tomas Berdych

Miami Open

Milos Raonic

Miami Open

Simona Halep

Miami Open

Victoria Azarenka

Miami Open

Jo-Wilfried Tsonga

Miami Open

Carla Suarez Navarro

Miami Open

Thanasi Kokkinakis

Miami Open

Stan Wawrinka

Indian Wells

Serena Williams

Indian Wells

Serena Williams

Indian Wells

Roger Federer

Indian Wells

Bernard Tomic

Indian Wells

Novak Djokovic

Indian Wells

Tomas Berdych

Rome

Eugenie Bouchard

Rome

Ana Ivanovic

Rome

Steve Johnson

Rome

Fabio Fognini

Rome

Novak Djokovic with his parents

Maria Sharapova

Rome

Novak Djokovic

Rome

Rafael Nadal

Rome

Caroline Garcia

Madrid Open

Serena Williams

Madrid Open

Rafael Nadal

Madrid Open

Sloane Stephens, Serena Williams

Madrid Open

Andy Murray

Madrid Open

Petra Kvitova

Madrid Open

Stan Wawrinka

Monte-Carlos Masters

John Isner

Monte-Carlos Masters

Rafael Nadal

Monte-Carlos Masters

Novak Djokovic

Monte-Carlos Masters

Tomas Berdych, Novak Djokovic

Monte-Carlos Masters

Stan Wawrinka

French Open

Lucie Safarova

French Open

Alize Cornet

French Open

Maria Sharapova

French Open

Taylor Harry Fritz and Tommy Paul

French Open

Luci Safarova and Bethanie Mattek-Sands

French Open

Timea Bacsinszky

French Open

Roger Federer

French Open

Novak Djokovic, Stan Wawrinka

French Open

Richard Gasquet

Wimbledon

Leander Paes and Martina Hingis

Wimbledon

Serena Williams

Wimbledon

Garbine Muguruza

Wimbledon

Serena Williams, Maria Sharapova

Wimbledon

Andy Murray

Wimbledon

Jelena Jankovic

Wimbledon

CoCo Vandeweghe

Wimbledon

John Isner

U.S. Open

Kevin Anderson

U.S. Open

Roger Federer

U.S. Open

Petra Cetkovska

U.S. Open

Nick Kyrgios

U.S. Open

Bethanie Mattek-Sands

U.S. Open

Donald Young

U.S. Open

Fabio Fognini

U.S. Open

Eugenie Bouchard

U.S. Open

Mardy Fish

U.S. Open

Belinda Bencic

U.S. Open

Serena and Venus Williams

U.S. Open

Roberta Vinci

U.S. Open

Roberta Vinci

U.S. Open

Novak Djokovic

U.S. Open

Roberta Vinci, Flavia Pennetta

U.S. Open

Flavia Pennetta

U.S. Open

Roger Federer, Novak Djokovic

U.S. Open

WTA Finals

Flavia Pennetta, Maria Sharapova

WTA Finals

Petra Kvitova, Chris Evert, Agnieszka Radwanska

WTA Finals

Novak Djokovic

ATP Finals

Rafael Nadal, Novak Djokovic

ATP Finals

Davis Cup Final

Andy Murray

Davis Cup Final

Kyle Edmund

Davis Cup Final

Oracene Price, Serena Williams, Venus Williams and Lyndrea Price

Sports Illustrated Sportsperson of the Year

Serena Williams accepting the 2015 Sports Illustrated Sportsperson of the Year award.

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.

Is the decline of Rafael Nadal imminent?

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?

SI's 2015 tennis awards: MVPs, disappointments, quotes and more

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.

How will Garbine Muguruza respond after her surge in 2015?

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.