ANDY MURRAY
Age: 29
Seeded: 1
Murray usurped Novak Djokovic for the No 1 ranking after a stunning finish to 2016, when he won Wimbledon and the Olympic gold medal at Rio de Janeiro and added titles in Beijing, Shanghai, Vienna and Paris. He clinched the year-ending No 1 ranking by winning the ATP Finals. He became the first British man to reach No 1 in the ATP rankings, the second-oldest player to debut at No 1. He set a record for most time between first gaining No 2 and No 1 rankings - 7 years and 82 days, after seven stints at No 2. After setting so many marks for British men, one still eludes him: an Australian Open title. He has lost 5 finals in the last 7 years at Melbourne Park, including 4 to Djokovic ('16, '15, '13, '11). He reunited with coach Ivan lendl in June last year.
NOVAK DJOKOVIC
Age: 29
Seeded: 2
Nobody has won more Australian Open titles than Djokovic, who made his Grand Slam breakthrough by winning the 2008 final as the No 3 seed. He Won three in a row from 2011-'13 and the last two - both against Murray. He excels on the hard courts at the Australian Open, where he is aiming to be the first man to win seven singles crowns and break a tie with Roy Emerson, who won his home title in 1961 and from 1963-67. He reached six consecutive major finals, winning five, from the 2015 Australian to winning his maiden French Open title last year. He lost in the third round at Wimbledon to Sam Querrey, then was beaten by Stan Wawrinka in the US Open final. He Spent 122 consecutive weeks at No 1 before being replaced by Murray on Nov. 7. He beat Murray in the final at Doha to open the season.
STAN WAWRINKA
Age: 31
Seeded: 4
The long-time No 2 even in his home country, Wawrinka won his maiden Grand Slam title at Melbourne Park in 2014, when he beat Nadal to become the oldest first-time major winner since Goran Ivanisevic in 2001. He has learned how to peak when it counts, beating the World No 1 in the finals at the Australian Open, the French Open (against Djokovic in 2015) and the US Open (Djokovic, last year). According to the ATP, he is 0-20 in other rounds and tournaments against the No 1 player. He broke the Djokovic-Murray run of majors when he won at the US Open last year, becoming the oldest men's champion at Flushing Meadows since Ken Rosewall in 1970.
ANGElIQUE KERBER
Age: 28
Seeded: 1
She had a breakout year in 2016 by winning her first Grand Slam singles title at Melbourne, beating Serena Williams in the final, but lost to Serena in the Wimbledon final and won a silver medal at the Rio Olympics and then captured her second major after beating Karolina Pliskova in the US Open final. She made seven finals and officially overtook Williams for the No 1 ranking on the Monday after the US Open ended, but was assured of the top ranking when Pliskova beat Serena in the semifinals in New York. She was a unanimous selection for WTA Player of the Year. She began 2017 by losing to Elina Svitolina in the quarterfinals at the Brisbane International, then was beaten 7-6 (5), 6-2 in her first match at the Sydney International by 26th-ranked Daria Kasatkina.
SERENA WIllIAMS
Age:35
Seeded: 2
It was always going to be difficult for Williams to match her stellar 2015 after winning the Australian Open, French Open and Wimbledon before losing in the US Open semifinals, falling just two matches short of the calendar Grand Slam. She had an off-year by her standards in 2016, winning only two titles, but one of them was her 22nd Grand Slam singles title, which she got at Wimbledon, equaling Steffi Graf's Open-era record. She ended the year without the No 1 spot for the first time since 2012. The newly-engaged (to Reddit co-founder Alexis Ohanian) Williams opened the year at the ASB Classic in wind-swept Auckland with a surprise second-round loss in three sets to fellow American Madison Brengle.
AGNIESZKA RADWANSKA
Age: 27
Seeded: 3
Melbourne Park marked Radwanska's best Grand Slam performance last year, losing in the semifinals to Serena Williams in straight sets. She lost in the fourth round at the other majors. She won two of her three tournaments in 2016 in China - at Shenzhen the week before the Australian Open and at Beijing in October. One of her nicknames is "The Magician" because of her shot-making ability, and she won the WTA Shot of the Year for the fourth year in a row in 2016. The shot came against Monica Niculescu at Indian Wells in March, when Radwanska ran all over the court during a rally before looping an around-the-post winner past her startled opponent.