U.S. Open 2015: Five things to watch

Updated 1056 GMT (1856 HKT) August 31, 2015
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Nick Kyrgios, left, received further punishment from the ATP this week for sledging Stan Wawrinka, right. Thus, Kyrgios is sure to be closely monitored in New York. Clive Brunskill/Getty Images Europe/Getty Images
Kyrgios has faced controversy throughout the year and at Wimbledon was fined $9,500 for two offenses. He was also accused of tanking, which he denied. Minas Panagiotakis/Getty Images
Wawrinka will be seeking his second major title of the year after winning the French Open in June. Clive Brunskill/Getty Images Europe/Getty Images
Rafael Nadal is struggling. He hasn't made the semifinals at any major this year and lost early -- for him -- at warmup tournaments in Montreal and Cincinnati. Maddie Meyer/Getty Images North America/Getty Images
Nadal knows what it's like to win at the U.S. Open, having triumphed twice, including on his last visit in 2013. Al Bello/Getty Images North America/Getty Images
The men's favorite is world No. 1 Novak Djokovic. He's won two majors this year and been in all three grand slam finals. If he wins in New York, he would reach double digits in grand slam crowns. Maddie Meyer/Getty Images North America/Getty Images
The heat affected Djokovic last year at the U.S. Open, when he was upset by Kei Nishikori. Nishikori would lose the final to Marin Cilic. Streeter Lecka/Getty Images North America/Getty Images
World No. 2 Simona Halep reached back-to-back finals in Toronto and Cincinnati after slumping at both Wimbledon and the French Open. Rob Carr/Getty Images North America/Getty Images
Can she, however, play her best tennis in the pressure cooker environment of a grand slam? In her last four grand slams, she's made the quarterfinals once. Julian Finney/Getty Images Europe/Getty Images
This is Eugenie Bouchard's last opportunity to salvage her season, at least at the grand slams. Bouchard has won nine matches all season and her ranking has dipped from fifth last October to 25th. Elsa/Getty Images North America/Getty Images
Maria Sharapova's late withdrawal with a knee injury ends prospects of her facing another semifinal against Serena Williams, who beat the Russian in her last outing at Wimbledon. ADRIAN DENNIS/AFP/Getty Images