Serena Williams celebrates her win against Justine Henin in the final in Key Biscayne on Saturday. (AP)

Key Biscayne: Serena Williams’ physical strength is there for everybody to see — the muscles in her arms and legs about as subtle as an overhead smash. While the eye is drawn to Serena’s brawn, her greatest asset goes largely unnoticed.

A rare opponent can push Serena to the point where it becomes obvious that her strongest muscle is her heart. Justine Henin was that formidable foil on Saturday in the final of the Sony Ericsson Open, with their riveting two-and-a-half hour match turning on Serena’s fearlessness in the face of adversity.

Serena, a former No. 1 player and the reigning Australian Open champion, lost the first set in 27 minutes and staved off two match points in the second before roaring back to win her fourth title 0-6, 7-5, 6-3 (as reported in Sunday’s Late City edition).

With the victory against Justine, the new world No. 1, Serena will ascend to No. 11 in the rankings to be released on Monday. After playing in only four events in 2006 because of injuries and burnout, the 25-year-old Serena started the year ranked No. 81.

“I kept fighting,” Serena said. “Even though I was down, I never was out.”

Justine, whose career, like Serena’s, has progressed in fits and starts because of injuries and illness, took a break at the start of this year to deal with the dissolution of her 4-year marriage. Justine, a 24-year-old Belgian, is considered a heavy-mettle player. But on this day, she was the one who blinked first.

After committing five unforced errors in the first set, Justine made 12 in the third as she wilted under the pressure that Serena applied.

“I felt I had the match under control and then the match turned over,” Justine said. “She played better than me on the important points. It’s tough to close the matches against her because she goes for it.”

What the match did not have in drama in the beginning it more than made up for in the final set and a half, which was characterised by high-level tennis.

The match was a study in contrasts, pitting players with a combined 13 Grand Slam championships who have different body types, games and temperaments.

Williams is 5 feet 9 inches, with the curves of a muscle car. At 5-6 and 126 pounds, Justine is as compact as the Mini Cooper she drives.

Serena overpowers opponents. Justine lures them into submission with patience, quickness and a fine array of shots.

But on Saturday, Serena showed plenty of finesse, winning more points at the net than Justine, and Justine displayed impressive power, finishing with more aces (five to Serena’s three).

Justine kept Serena off balance in the first set with her impressive serves and the depth and breadth of her shotmaking. Serena had a hard time keeping the ball in play, with the backhand she sent wide on set point epitomising her early troubles. Serena had 14 unforced errors in the first set to Justine’s 5.

The match turned in the 10th game of the second set, when Justine had two championship points on her serve, at 40-15 and 40-30. She knew better than to count out a cornered opponent, having herself come back from a 5-1 deficit in the third set of her third-round match against Virginie Razzano to prevail in a tie breaker.

Serena had spoken earlier in the week of Justine’s returning to the Tour after a self-imposed absence and catching ‘that Serena flame’, adding, “I’m trying to catch it, too.”

She did in the nick of time, staving off both championship points with big forehands and beautifully constructed points. Serena broke Justine, and won the next five games as well.

In that stretch, Justine twice hurt her knees lunging for shots. Whether it was a muscle strain or the mental strain of trying to close out Serena, Justine did not cover the court with the same fluidity in the third set that she had exhibited in the first.

Justine fought back to tie the third set at 3-3. Serena, serving at 40-0 in the next game, hit a cotton ball of a drop shot that Justine returned, and then passed her with a backhand for one of her 10 winners in the set (she had four in the first).

“I feel like when I get down,” Serena said, “a part of me just plays better.”