In Tuesday's final match of the day, the Australian's title defense came to a crashing halt, when he retired after dropping the first set, 6-3, to Ugo Humbert, due to a left wrist injury. The issue came up a week ago in Delray Beach, when he withdrew for the second straight week from an ATP event. He previously pulled out of the New York Open with a shoulder problem.
Krygios was playing his first tournament since losing to Nadal in the round of 16 at the Australian Open. Trailing 1-4, the No. 6 seed called the trainer to come tape his left wrist. “Just tape it,” Kyrgios said, “I’m going to pull out anyways.”
Four games later, the 24-year-old threw in the towel and was booed as he exited the court. Ranked No. 23 entering this week, Kyrgios could find himself outside of the Top 40 as a result, depending on the rest of this week's outcomes.
Asked about the booing, the Australian said he didn't "give a f***" about "their disrespect" and had done what he could to compete. Kyrgios said his wrist was "not ready" for competitive play. He was not hitting backhands properly during the first-round encounter.
"I tried to play. I've been doing media," he said. "I tried to give the fans a little bit of tennis."
WATCH—Kyrgios retires in Acapulco:
Nadal and Zverev enjoyed much more pleasant returns to the court in winning their openers over Pablo Andujar and Jason Jung.
The top-seeded Nadal, who last played at Melbourne Park in a losing effort to Dominic Thiem during the quarterfinal stage, dismissed Andujar, 6-3, 6-2. The world No. 2 triumphed at this event in 2005 and 2013, when it was staged on red clay.
Seeded No. 2, Zverev pulled away from qualifier Jung to advance, 7-6 (6), 6-1, after 72 minutes.
"The first set was a little bit shaky, but that's expected. The last few years when I did well here, I also had some scrappy starts," Zverev told Prakash Amritraj. "I didn't play my best tennis in the early rounds and got better as the tournament went on. I hope it will be the same case this time."
WATCH—Zverev talks with Tennis Channel's Prakash Amritraj
Zverev reached his first major semifinal at the Australian Open where Thiem also dashed his title hopes. The German will meet Tommy Paul next.