Top seed Azarenka retained her Qatar Open WTA title with a 7-6 (8-6) 2-6 6-3 win over Serena Williams.
The Belarusian will lose the world No.1 ranking to the veteran American on Monday and she went into Sunday's final having lost her last nine meetings with Williams.
Her win was only her second in 13 career meetings with the American.
Nadal clinched his first tournament title since ending a seven-month knee injury layoff in winning the ATP Brazil Open.
The Spanish former world No.1, who has been complaining of nagging knee pain, beat Argentine David Nalbandian in straight sets 6-2 6-3.
Nadal was the top seed in the tournament, his second comeback event after a seven-month absence.
He competed in the Vina del Mar Open in Chile earlier this month, losing the singles and doubles finals last Sunday.
Considered by many to be the best ever clay-court player, Nadal was back on the court in Chile for the first time since a surprise second-round exit at Wimbledon in June.
Since then he has been sidelined by a torn tendon and inflammation in his left knee, with his return this year further delayed by a stomach virus.
The world number five, who has 11 grand slam titles under his belt, also won the Brazil Open in 2005, when it was held in Costa do Sauipe in eastern Bahia state.
Last year, the tournament was moved to Sao Paulo.
Del Potro defeated Julien Benneteau 7-6 (7-2) 6-3 to win the Rotterdam World Tennis and deny the Frenchman's dream of a first career title.
Del Potro played the last game with cotton wool stuffed up one nostril due to a bloody nose which came on after the previous game in which Benneteau saved four match points.
But the Argentine second seed, who lost the final here a year ago to Roger Federer eventually defeated Benneteau, Federer's conqueror in the quarter-finals, with 13 aces and three breaks of serve.
Del Potro became the second Argentine to win in Rotterdam after Guillermo Vilas in 1982.
Benneteau has now lost in all eight finals he has played.