The ATP Tour officials say they are reviewing "allegations of racial prejudice" after Ryan Harrison and Donald Young exchanged words in the first set and had to be separated by the chair umpire during a changeover in play on Monday.
The 6-4, 6-2 win was Nick Kyrgios' fourth title on the ATP tour and his first on home soil.
The US Open runner-up broke Harrison five times to advance to the third round of the WTA and ATP Masters hardcourt tournament.
Hewitt, the 32-year-old Australian who once topped the world rankings, made his first start in the event since 2009.
Novak Djokovic was in the zone with a dazzling demolition of Ryan Harrison as he stormed into the third round of the Australian Open on Wednesday.
Defending champion John Isner hammered 12 aces to defeat Izak van der Merwe 6-4, 7-6 (7/2) and reach the semi-finals of the ATP Tour's Hall of Fame Tennis Championships on Friday.
French number one Jo-Wilfried Tsonga will bid to get the hosts off to a winning start against the United States when he plays teenager Ryan Harrison in their Davis Cup quarter-final clash here on Friday.
Roger Federer set up a 24th career meeting against old rival Andy Roddick on Saturday as both men eased through the Miami Masters second round, but crowd favourite Kim Clijsters crashed out of the women's event.
Milos Raonic put on an impressive serving display with a straight set semi-final win over Ryan Harrison at the San Jose Open, avenging a loss to the American in their only previous contest.
Defending champion Milos Raonic of Canada advanced to the semi-finals of the $531,000 ATP San Jose Open on Friday by defeating South African sixth seed Kevin Anderson 7-5, 7-6 (7/3).
British fourth seed Andy Murray bounced back from losing the opening set to oust American Ryan Harrison in four sets and reach the second round of the Australian Open on Tuesday.
Fifth-seeded Nikolay Davydenko of Russia beat Ryan Harrison of the United States 6-3, 6-2 on Tuesday to advance to the second round of the Malaysian Open.
Former champion Lleyton Hewitt, Robby Ginepri, Ryan Harrison and Donald Young are among the seven players awarded wild cards into the men's draw at the U.S. Open.
Rising American prospects Ryan Harrison and Laura Davis each won a spot in next month's Australian Open at a wild card playoff tournament in the United States.