Smith beat Pratt 6-3, 6-1 at the US Open two years ago and made another fine start. But she then lost four games in a row to be a set down and lost her serve again at 4-4 in the second.
After saving two match points the 27-year-old Essex player volleyed over the baseline with the whole court at her mercy and Pratt, ranked 102nd against Smith's 64th, won when yet another forehand flew out.
"I'm really disappointed," she said. "I never thought I was going to lose, but I missed so many open court shots.
There was more disappointment for British fans in a record crowd when the doubles pair Neil Broad and Andrew Richardson lost in straight sets in the first round to France's Olivier Delaitre and Fabrice Santoro. The seventh seeds won 6-7, 6-3, 6-3.
In the women's doubles Britain's Julie Pullin teamed up with the Czech Republic's Eva Melicharova, but they had the misfortune to meet Martina Hingis and Anna Kournikova. Predictably, they lost 6-3, 6-0 to the No 3 seeds.Reuse content