OKLAHOMA CITY -- Venus Williams defended her title and joined her sister in making history Sunday, defeating Amanda Coetzer in straight sets at the IGA SuperThrift Tennis Classic.

Earlier in the day, in Paris, Serena Williams won the Gaz de France Open, making the Williams' the first sisters to win WTA Tour events on the same day."I found out that she won before I came out to play the match, so I really felt that it was my duty to come out here and win," said Williams, who had been trading e-mail messages with her sister during the week.

Williams, ranked No. 5 in the world, defeated Coetzer 6-4, 6-0. She didn't lose a set in any of her four matches this week, and none of the four lasted more than about an hour. Sunday's took 58 minutes.

Williams broke Coetzer in the first game of the day and the two stayed on serve the rest of the way. She had seven aces, including three in the final four points of the set.

The second set lasted just 21 minutes. Williams only had three aces, but took advantage of the South African's mistakes.

"I don't think I played so well in the first set," Williams said. "I think my unforced error count was a bit high. But maybe Amanda's the type of player that makes you hit some errors.

"I always feel that there's no need to make a mistake, so the second set was not a surprise for me."

The 18-year-old Williams won her first pro title at this event last year. Sunday's victory, her fourth as a pro and first this year, was worth $33,000.

Serena Williams, 17, beat Amelie Mauresmo 6-2, 3-6, 7-6 (7-4) to become the first American to win that tournament. Her victory was worth $80,000.

Serena Williams had never played a WTA Tour singles final. She has played three doubles finals with her sister and won all three.

Coetzer was a finalist at this event in 1996, losing to Brenda Schultz-McCarthy.

Serena Williams wins first title

PARIS -- For Serena Williams, intent on climbing tennis' summit, this was a start.

The 17-year-old won her first title on the WTA Tour on Sunday, beating Amelie Mauresmo 6-2, 3-6, 7-6 (7-4) in the final of the $520,000 Gaz de France Open.

"I have always dreamed of winning Grand Slams and this is a start," Williams said. "It is good to win a smaller tournament because when I get to the big events I will have the experience."

A continent away, Serena's older sister was taking care of family business. Venus captured the IGA SuperThrift Tennis Classic at Oklahoma City with a 6-4, 6-0 victory over Amanda Coetzer, marking the first time sisters have won on the WTA Tour in the same week.

Serena Williams, after letting a 4-1 lead get away in the third set, was unable to close out Mauresmo when she served for the match at 5-4. But the 17-year-old regrouped in the tiebreaker, firing an ace at 5-3, and became the first American-born player to win this tournament.

The Gaz final matched two imposing and muscular players. Williams and Mauresmo were both bidding for their first tournament title.

Williams, who was not seeded, beat two seeds -- Nathalie Tauziat and Julie Halard-Decugis -- en route to the final. She earned $80,000 for the victory.

Mauresmo, the No. 6 seed, enjoyed a big quarterfinal victory on Friday over Martina Hingis in a rematch of the Australian Open final.

Williams had never played a WTA Tour singles final before. She has played three doubles finals, with sister Venus, and won all three.

"My technique broke down so I wasn't making the shots I should have been making," Williams said. "I didn't feel so good at 5-6 behind, but I told myself that if I want to be the best I need to learn how to close out matches."

As she had against Hingis, Mauresmo showed she could lift her game after losing the first set. The 19-year-old Frenchwoman took the second set when Williams double-faulted. From there, Williams moved to a 4-1 lead in the final set before the tiebreak.

Williams, whose world ranking improves from No. 24 to No. 21, said Mauresmo has powerful strokes, but nothing like her sister's.

"I didn't think that (Mauresmo) hit that hard," she said. "But maybe I am just used to it."