Watson won just two service games on her way to a 6-3 6-4 defeat to Belgium's Elise Mertens.
It was the sixth time Watson had reached the second round at Roland Garros, but she has still yet to advance any further.
Watson had breezed through her first-round match against France's Oceane Dodin, dropping just three games along the way. She also hit nine aces.
But the 26-year-old's serve deserted her this time, with five double-faults added to 37 unforced errors giving her a mountain to climb against anyone, let alone the 16th seed.
Not that Mertens was much better. The first set featured a staggering eight breaks of serve in nine games.
Mertens secured another 'crucial' hold at the start of the second before breaking Watson yet again.
Watson finally won on her own serve at the eighth attempt, and forced her way to back on serve at 3-4.
Both players held again but Watson had the task of serving to stay in the match, and given what had gone before it was not a surprise that she could not manage it.
"There were just too many errors today, and I was very up and down," said Watson.
"I would get on a hot streak and win a couple games, and then I would just make four unforced errors, and that needs to change.
"Also my serve. I had a low first-serve percentage and wasn't getting as many free points as I would have liked.
"The other day I served better than I normally serve. So I wouldn't say that's how I normally serve. I would say that was a very good day.
"It's just different conditions. Also, it's a different opponent. It's different serving after playing a 10-ball rally than just serving after somebody's just missed your return. So it's just a different situation."