FUTURE world number one Martina Hingis rose to 25-0 on the WTA Tour this year and reached the finals of the Lipton Championships by outlasting third seed Jana Novotna at Key Biscayne yesterday. In extreme heat, the young Swiss player overcame her Czech rival, 6-3, 2-6, 6-4.

Hingis will become the youngest world number one in tennis history on Monday when she replaces injured Steffi Graf on top of the WTA rankings. She will be 16 years, six months and one day - nine months and 18 days younger than Monica Seles when she set the standard in 1991. Hingis was ranked 399th three years ago when she joined the tour.

Hingis has not lost since dropping a five-set duel with Steffi Graf last November at the WTA Championships in New York. Her two losses came just before that to Novotna in a final in Zurich and a semi-final in Chicago. She has won four titles in a row. She captured her first Grand Slam crown at the Australian Open and also won in Sydney, Tokyo and Paris.

Yesterday, Novotna made 49 unforced errors and two key errors in the ninth game of the third set turned the match. She double faulted to give Hingis a break chance and sent a drop volley into the net to fall behind 5-4.

Hingis received her first match point when Novotna sent a drop volley into the net and won by serving the match's only ace on the final point.

Players took a break before the third set under WTA extreme heat rules as temperatures reach 107 degrees F (42 C) under the midday sun.

Hingis hurled her racket under her bench after sending a forehand wide to give Novotna the opening game of set three after squandering a break point.

Hingis then held serve in four points, the final two on drop volleys, and broke Novotna in the third game, sending a backhand winner past on her third break point opportunity of the game.

Novotna broke back in the next game by saving two overhead smashes, the second with a backhand passing winner that had the Czech pumping her fist into the air.

On the next point, Novotna made a falling lunge volley at the net for a winner, then curtsied to the cheering crowd. She went on to hold for a 3-2 lead.

A frustrated Hingis struggled to hold serve in the eighth game of the last set, bouncing her racket and double faulting to give away a break point before holding to 4-4 on a cross-court volley and fore-hand winner.

A double fault and too-long lob by Novotna gave Hingis a service break in the second game. Novotna broke back with a forehand winner in the seventh but was broken back in the eighth when she hit a forehand into the net.

Novotna answered in the second set, coming to the net quickly and often to break Hingis in the fourth and final games.

Novotna, who fell to 2-3 overall against Hingis, will rise to third in the new rankings, matching her career high.

Meanwhile in the men's singles, at Key Biscayne, former world number one Jim Courier continued an unlikely run in his hometown event, advancing to the semi-finals. Courier ranked 26th upset fourth-seeded Croatian Goran Ivanisevic 6-2, 7-6 (7/2).

Courier, who lives on nearby Fisher Island, took his fourth victory in a row over Ivanisevic and his sixth in eight meetings overall. Ivanisevic has not beaten Courier since 1992.

The win gave Courier, who has not been number one since 1993, back-to-back victories over top 10 opponents for the first time since the 1995 US Open. Courier beat Richard Krajicek here in the fourth round.

Courier opened the year by winning in Doha for his 20th career crown, but had not advanced past the second round in his past three tournaments.

Ivanisevic, who quit last years, Lipton final with a stiff neck, as never won a US ATP title. He leads the tour in aces and victories this year and has already defended titles this year in Milan and Zagreb.