The 23-year-old American advanced to her first final since a six-month layoff with a strained abdominal muscle following her loss to younger sister Serena in last year's Wimbledon final.

"I feel great about getting to the finals of this event, my first clay court of the year, first final of the year," said the former No. 1, who is making her debut at this tournament.

But Williams, who will be seeking her 30th career title, had to hustle in the first set as her left-handed opponent delivered a potent mix of shots including some well-disguised drop-shots.

"When I'm playing good, my drop shots are probably pretty good, and a lot of players struggle with them," said Kostanic, who scored her best Tier I result this week.

"I just know I have that kind of variety where I can play long slice, short slice, drop shot, and I'm not scared to play it."

In the second set, experience pulled through and Williams served a 106 mph ace for the victory on her fourth match point.

Martinez fired three aces and 16 winners to defeat Switzerland's Patty Schnyder 6-4, 6-3 and will try to deny Williams her first title in 14 months.

Martinez, who rose to 7-2 against Schnyder, turned 32 on Friday and on Monday captured her 700th career match victory.

"Hopefully Venus is not going to to out there and overpower me in two shots, so I'm going to try to just get to the balls, fight like crazy and try to do whatever I can with my shots, like hitting different slices, and also try to be aggressive," Martinez said.

"You can't be too passive because she hits the ball really hard."

Reigning Wimbledon champion Serena Williams withdrew from the event with a sore left knee and Belgian world number Justine Henin-Hardenne, the defending champion, pulled out due to illness and will miss at least two more weeks.