But by the time the game started, the Toronto Raptors forward felt even worse.
And, for three quarters, it showed as Gay shot a woeful three-for-13 from the field. However, he had 11 points in the final 12 minutes of play as Toronto used a dominant fourth quarter to defeat the Washington Wizards 96-88 to remain in first place in the Atlantic Division.
Toronto trailed Washington 70-66 heading into the final quarter, but Gay hit all three field goal attempts in the fourth and added five free throws to lead the Raptors to victory.
"There was no stopping me. I was going to play," said Gay. "I was out of breath and the sort of things that happen when you're sick but I had to push through it for my team.
"In the fourth quarter, I just let it come to me and it got a lot easier."
Gay's 11-point fourth quarter gave him 17 for the night, tied for team high with DeMar DeRozan, as Toronto (6-7) won for the second straight time to stay atop the NBA's Atlantic Division.
"He struggled a little bit early but when it was down the stretch, he came through for us," head coach Dwane Casey said of Gay, who was booed in the third quarter by the 18,671 in attendance for his poor shooting performance and a couple of turnovers. "He was a little weak from the flu but he gutted it out.
"He could have easily mailed it in or checked out but he fought through it and gave us 17 big points."
Toronto scored the first nine points of the fourth quarter to wrest the lead back from Washington. The Raptors outscored the Wizards 21-9 over the opening 7:34 of the final period and 30-18 overall.
It was a dramatic turnaround from the third quarter when the Wizards (4-8) outscored Toronto 32-15, turning a 13-point halftime deficit into a four-point lead going into the final quarter.
Washington point guard John Wall, who had a game-high 37 points, scored 18 in the quarter.
"Teams are going to go on runs," said Toronto forward Tyler Hansbrough, who had eight points and six rebounds off the bench. "As we continue to grow, we can't have lapses where it carries over and have a hangover until the fourth quarter.
"We've got to keep on moving and that's what happened."
The Raptors, who shot 47 per cent from the field on the night, featured a balanced attack, with six players finishing with double figures in points. Jonas Valanciunas had 11 points and 13 rebounds.
"Third quarter, we have to figure that out sooner or later," said guard Kyle Lowry, who had 12 points, nine assists and six rebounds on the night. "(But) that group that came in in the fourth did a great job getting us a 10-point lead.
"They (Wizards) made a run at the end but we closed it out."
Toronto returned to Air Canada Centre to open a four-game homestand — their longest of the season — leading the division for the first time since they won it in 2007.
And the Raptors came out flying, shooting an eye-popping 67 per cent from the field in the first quarter to take a five-point lead, which they widened to a 13-point edge at halftime.
It appeared the Raptors were on the way to an easy victory before Wall, the first overall draft pick in the 2010 NBA draft, who signed a five-year, US$80 million contract extension in July, went off.
"You can see why he got paid all that money," De Rozan said. "He's definitely a heck of a player.
"He has the passion to win and he showed that in the third quarter to try and bring his team back.
"We had to withstand that as much as we could and get the win."
The Raptors return to action Tuesday hosting Brooklyn before closing out the homestand with games against Miami and Denver.