Petrova, ranked No. 10 in the world, won nine of the last 11 games to beat Keys, who was playing in her first WTA Tour event in the season-opening clay-court tournament.
“At the beginning of the match I didn’t know what to expect,” the Russian said. “She caught me by surprise.”
The match was tied 3-3 in the first set before Petrova won three straight games.
“I had to make some adjustments,” she said. “I was trying to go for too many winners early, trying to blow her away and just had to slow down.”
Despite the loss, Keys was pleased with her play.
“When I hit a good shot she hit a good shot back,” she said. “I feel pretty good about going 3 and 2 with a top 10 player.”