Andreev, seeded seventh, beat Nicolas Kiefer of Germany 5-7, 7-6 (3), 6-2. Pierce downed Francesca Schiavone of Italy 6-4, 6-3.
"It's unbelievable. The dream of my childhood has come true," Andreev said.
The Russian, who won his first two ATP titles earlier this season, led Kiefer 5-3 in the first set before losing eight straight games.
He fell on the court in the second game of the second set, called for a trainer after the second and third games to treat his left leg and took a medical timeout after breaking Kiefer at love in the fifth game.
Andreev then won three consecutive games to level 4-all and hit his eighth ace to win a tiebreaker and stay in the match. Kiefer then took a medical timeout after the second set to treat his feet.
An early break put Andreev ahead 3-0 in the third set and he sealed the victory with a forehand return on Kiefer's serve in the eighth game.
The 30-year-old Pierce won the 18th title of her career.
Schiavone, ranked 22nd, had upset second-seeded Amelie Mauresmo, eighth-seeded Svetlana Kuznetsova and fourth-seeded Elena Dementieva on the way to her fourth career WTA Tour final.
The Italian, who has not dropped a set in her four matches, was broken twice in the first set, giving Pierce a 4-1 lead.
Schiavone broke back in the eighth game and had a break point at deuce in the 10th, but Pierce served three winners to take the set.
"She has done so well this week and had great success, so it wasn't easy," Pierce said. "In the beginning she played really well. I didn't play great tennis but I had some good shots and it was important for me to play smart,"
After an exchange of breaks, Pierce broke the Italian in the fifth game of the second set and sealed the win with an overhead smash on her first match point.
"She made a great job today," Schiavone said. "She put so much pressure on me and didn't give a chance to come back."
Pierce won US$189,000 (¤157,513) and will move to No. 5 in the WTA Tour rankings for the first time since October 2000, when an injury forced her to retire in the fourth round of the U.S. Open and miss the rest of the season.
The win will also move her into fourth place in the race for the season-ending WTA Tour Championships in Los Angeles in November. U.S. Open champion Kim Clijsters, Lindsay Davenport and top-ranked Maria Sharapova are already in the field.Reuse content