Nick Kyrgios risked punishment from the ATP after once again inviting criticism of his conduct at the Shanghai Masters.
The Australian - fined and suspended by the Tour for 'tanking' in a match against Mischa Zverev at this event last year - retired from his first-round meeting with Steve Johnson at the end of the first set, despite showing few signs of discomfort.
During the tie-break, Kyrgios was 4-2 ahead but could be heard complaining about fatigue as well as declaring his intention to forfeit if he went a set down.
He was given a point penalty for swearing and, after Johnson sent an ace down the T to win the breaker 7-5, Kyrgios shook hands with his opponent at the net and left to the boos of the crowd.
Kyrgios subsequently took to Twitter to explain he had been battling a stomach bug over the previous 24 hours, but hoped to resume his doubles campaign alongside Lucas Pouille on Wednesday.
SCHWARTZMAN TO FACE FED
With a first-round bye, 2014 champion Roger Federer enjoyed Tuesday off as Diego Schwartzman and Jordan Thompson battled it out for the right to face the Swiss.
And it was the former who advanced 6-4 7-6 (7-4) after patiently grinding down the resistance of his Australian opponent.
Thompson rescued nine break points across his opening two service games, but it was 10th time lucky for Schwartzman, who broke for 3-2 and served out for the lead.
World number 72 Thompson staved off two more break chances at 5-5 in the second but his respite was brief as Schwartzman claimed the tie-break and a second-round meeting with Federer.
CILIC AVOIDS A SLIP-UP
Marin Cilic faced a tricky opponent in Kyle Edmund, but the fourth seed had more than just the world number 53 to contend with as conditions proved difficult for both players.
The world number five broke twice in the first set - including in the opening game - to forge ahead before an evenly-matched second headed for a tie-break.
During the breaker, both players complained of a slippy surface behind the baselines, Edmund taking a nasty tumble as Cilic went 5-4 up. The Briton got to his feet to win the next point, but a forehand into the net set up match point and Cilic's assured volley sealed a 6-3 7-6 (7-5) success.
Alongside Cilic, four other players booked their places in the last 16 on Tuesday.
Albert Ramos-Vinolas got the better of an all-Spanish affair, defeating seventh seed Pablo Carreno Busta 7-5 7-5 while Fabio Fognini ousted Pouille 7-6 (9-7) 6-3.
Juan Martin del Potro overcame Andrey Rublev 6-3 6-4 and Richard Gasquet fought back to see off Hyeon Chung 3-6 6-4 6-3.
Meanwhile, in the opening round, 12th seed John Isner saved a match point at 6-5 down in his deciding-set tie-breaker with Dusan Lajovic, winning three points on the bounce to advance 4-6 7-6 (7-5) 7-6 (8-6).