Two other men widely considered to be in the next generation of major winners advanced in straight sets, with No. 5 Kei Nishikori beating Nicolas Almagro 6-4, 7-6 (1), 6-2 and No. 8 Milos Raonic firing 30 aces in a 7-6 (3), 7-6 (3), 6-3 win against qualifier Illya Marchenko.

No. 9 David Ferrer beat Thomaz Bellucci 6-7 (2), 6-2, 6-0, 6-3, No. 12 Feliciano Lopez struggled past American wild-card entry Denis Kudla 3-6, 6-2, 4-6, 6-2, 10-8, while No. 13 Roberto Bautista Agut, No. 18 Gilles Simon -4 and No. 19 John Isner also advanced.

No. 16 Fabio Fognini lost in four sets to Alejando Gonzalez.

Nishikori, returning to Grand Slam action for the first time since his run to the US Open final, dropped his opening service game but recovered the break quickly and weathered a difficult opening match against Almagro.

Nishikori is coming off a big year in 2014, when he became the first Asian man to reach a Grand Slam singles final. He's determined to go one better and win a major.

"Yeah for sure that was one of my best results in my career that gave me a lot of confidence," the Japanese star said. "I beat a lot of top 10 players."

Wimbledon champion Petra Kvitova and No. 6 Agnieszka Radwanska needed only 63 minutes for their straight-sets wins over Richel Hogenkamp and Kurumi Nara, and No. 18 Venus Williams beat Maria Teresa Torro 6-2, 6-2.

Two-time champion Victoria Azarenka continued her comeback from a foot injury with a convincing 6-3, 6-2 win over Sloane Stephens and will next have to play US Open finalist Caroline Wozniacki, who beat 18-year-old American Taylor Townsend 7-6 (1), 6-2.

No. 11 Dominika Cibulkova, a finalist here last year, opened with a 3-6, 6-3, 6-1 comeback win against Kirsten Flipkens.

Limited to just nine tournaments last year, Azarenka's ranking has fallen to No. 44 and she is unseeded at a Grand Slam for the first time since the 2007 US Open.

2011 US Open champion Sam Stosur had a 6-4, 6-1 victory against Monica Niculescu, and No. 19 Alize Cornet beat Zhang Shuai 6-3, 6-2. No. 24 Garbine Muguruza and No. 25 Barbora Zahlavoa Strycova advanced with straight-sets wins.