Ai Sugiyama's hectic day was highlighted by the biggest victory of her career.

After winning a gruelling semi-final match that lasted more than two hours earlier in the day, Sugiyama overcame doubles partner Kim Clijsters of Belgium in Sunday's final to win the State Farm Women's Tennis Classic.

Due to rain on Saturday, Sugiyama needed to stave off three match points against American Alexandra Stevenson to post a 6-7 (2-7) 6-2 7-6 (9-7) semi-final victory in a match that took two hours, 12 minutes.

After just 90 minutes of rest, Sugiyama came back to the Fairmont Princess Hotel centre court to upset the second seed and third-ranked player in the world 3-6 7-5 6-4 in two hours, four minutes.

"I felt tired when I was playing that second match, but actually it worked good," Sugiyama said.

"I didn't feel any added pressure because I was too tired.

"I just focused on every point. That helped me, I think. Even if I was feeling tired, I had to push myself, 'Just one point, one point.' ... I don't know how I pulled it out."

Sugiyama claimed her fourth WTA Tour singles title - her first at a Tier II event.

"Last time I won a tournament was Tier III in Japan in 1998, so it was a while since I won a title," she said. "This is Tier II and a big tournament, so many good players playing up there. I just couldn't believe it."

The 26-year-old from Japan will move up in the WTA rankings from number 25 into the top 20 when the rankings are released on Monday.

Sugiyama improved to 3-2 lifetime against Clijsters, handing her just the third loss in her last 16 matches.

Sugiyama showed her mental toughness when Clijsters was serving for the match at 5-4 in the second set.

"Even though she had a match point, I just worked on every moment," she said. "Stevenson played good and Kim has also been playing very good, so I had nothing to lose. I've been playing good, too. I just wanted to have fun out there."

Sugiyama broke on her third chance when Clijsters sprayed a forehand wide to level the set at 5-5.

She held to go ahead, 6-5, before Clijsters double-faulted on set point to level the match.

After taking the second set, the first lost by Clijsters in the tournament, Sugiyama got a break during a 45-minute rain delay at the start of the third set.

"It was a good break for me," said Sugiyama, who underwent treatment for a blister on her left foot and had a snack.

"Sometimes when I win the second set and I'm on a good rhythm, I want to start out good. But it was only two points into the third set, which was good timing."

When play resumed, Sugiyama broke in the third game, going ahead 4-1 when Clijsters smacked a forehand wide.

But the Belgian returned the favour in the seventh game and leveled the set at 4-4.

Sugiyama held serve with a service winner to go up 5-4 and had match point at 30-40 after Clijsters pushed a forehand drop shot wide. She closed out the shocker when Clijsters drilled a cross-court forehand over the baseline.

"I feel exhausted but great," Sugiyama said. "Even though I had to prepare for a 10 am match, I woke up at 7. It's been a long day, but it's been a good finish. It gives me a good fighting spirit. I'm still alive."

Clijsters was denied her 12th WTA title and second of the year.

"It's always a little frustrating when you're trying hard and things don't work," she said. "But that's tennis.

"I played so many matches where I'm playing players not as good as Ai, but then I can still get through it mentally. I felt I was working hard for every point and it can get a little frustrated."