Novak Djokovic puked on the Arthur Ashe Stadium court, cramped up, cried, and lost a grueling 5-set match just two days ago ... so, when we saw the tennis GOAT out in NYC, we asked the obvious question -- how are ya feeling?!

"Better. Bit better. A bit better," the 39-year-old tennis legend told TMZ Sports out in the Big Apple on Tuesday ... less than 48 hours after he battled illness during the 4-hour, 36-minute marathon against unseeded Argentine Mariano Navone.

It was the first time Djokovic lost in the opening round of the U.S. Open ... but it wasn't as simple as him playing bad tennis.

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Novak was sick -- he vomited multiple times during the match.

The New York crowd obviously saw he was struggling, and they did all they could to will the 24-time Grand Slam champ to victory, with thousands of fans chanting his name.

Even with the stadium behind him, it wasn't enough for Djokovic on Sunday ... but that doesn't mean the gesture wasn't greatly appreciated.

"Always love. Always love, and gratitude for the support," Novak told us.

Bottom line for the tennis legend -- he's not 100% just yet -- but he's on the mend!