DALLAS, May 24 (Ticker) -- The Dallas Mavericks must be wondering how long they have to pay for allowing Steve Nash to leave for the Phoenix Suns.

Nash piloted a late comeback capped by Boris Diaw's basket with 0.5 seconds left that gave the Suns a thrilling 121-118 victory over the Mavericks in Game 1 of the Western Conference finals.

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Diaw scored a career-high 34 points and Nash added 27 and 16 assists for the second-seeded Suns, who looked tired at times but relied on their resiliency and won a road game for the third straight series.

"We feel like we've overcome a lot of adversity this year to be where we are and we want to see how good we can get," Nash said.

"Every time we hear something about us, it's yeah, we are underrated," Diaw said. "We'll take the games one by one and see what's going to happen."

It was a disappointing and virtually inexcusable loss by the fourth-seeded Mavericks, who led by nine points with less than four minutes to play against the defenseless Suns but could not close it out.

"We didn't execute down the stretch," Mavericks guard Jason Terry said. "We had a nine-point lead with three-something left. A couple of turnovers, a couple of blown defensive assignments, execution on both ends of the floor, all correctable errors. It's disappointing we let one slip away.

The series opener was a continuation of the nail-biters that have pervaded the postseason. It was Phoenix's fifth game decided by less than three points or in overtime. Dallas has played six such games.

"Another normal night in the NBA," Suns coach Mike D'Antoni said.

Nash played five seasons in Dallas and was a fan favorite before leaving for Phoenix as a free agent in the summer of 2004. He has won a pair of MVP awards in two seasons with the Suns and victimized the Mavericks in last year's conference semifinals.

A driving three-point play by Devin Harris, who scored a career-high 30 points on 12-of-17 shooting, gave the Mavericks a 114-105 lead with 3:43 to go. But Dallas stumbled, and the NBA MVP pounced.

Nash scored Phoenix's next 10 points, making two free throws and a 3-pointer before going underneath good buddy Dirk Nowitzki for a difficult layup and over him for a 3-pointer that cut the deficit to 116-115 with 1:04 remaining.

"For much of the game, my teammates were playing great and making shots," Nash said. "I was trying to facilitate their games. The later it got in the game, I didn't want to pass up shots, especially with our having fallen behind."

On Phoenix's next possession, Nash ran a pick-and-roll with Shawn Marion, who dunked for the lead with 43 seconds left. The teams traded empty possessions before Harris made a 17-footer from the right wing, giving the Mavericks a 118-117 lead with 4.8 seconds to play.

After a timeout, Tim Thomas inbounded to Diaw, who had missed badly on the previous possession. He got deep position on Jerry Stackhouse and made a turnaround 10-footer from the right baseline, silencing the sellout crowd at the American Airlines Center.

"That wasn't a play for me," Diaw said. "From the start of the play, I was more of the second option. We ran a play we've run two or three times during the year for Steve. Dallas saw the play and I think they knew what we were going to do. I had the ball, so I had to take the shot."

In the final second, Marquis Daniels threw away the inbounds pass and Thomas sealed it with two free throws. Dallas had four turnovers in the final 77 seconds.

"The last couple of minutes, everything that could go wrong, did go wrong," Nowitzki said. "We couldn't get a stop. Nash made a couple of big shots. Diaw makes the tough turnaround and on the offensive end, our execution was horrible. We didn't even get a shot off until all the way at the end when Devin made the shot to get up a point, but it just wasn't what we were looking for. It's a disappointing loss and a tough way to start a series at home.

Marion sprained his left ankle in the fourth quarter but had 24 points and 13 rebounds and Thomas scored 17 points for the Suns, who shot 55 percent (47-of-85) from the field.

Nowitzki had 25 points and 19 rebounds and Stackhouse scored 16 points for the Mavericks, who shot 51 percent (47-of-93) and held a 48-38 advantage on the glass.

"Tonight we weren't very good," Mavericks coach Avery Johnson said. "Even though we had the lead, we couldn't execute. We were just bad tonight in a lot of different areas that we have to improve on in a hurry. Defensively, Nash - too easy. Diaw - too easy. I know they're really good players, but we've just got to play better defense."

Both teams lost key players to injuries. Mavericks forward Josh Howardsuffered a sprained right ankle midway through the first quarter and did not return. Suns guard Raja Bell suffered a left calf strain midway through the final period and was carried off the court.

"I thought someone threw something from the stands," Bell said. "But then it started to burn and right now it's pretty sore."

Perhaps now the Mavericks know how the San Antonio Spurs felt. After using their advantage in speed to dethrone the champions, Dallas had a hard time keeping up with Phoenix, the fastest team in the league.

As expected, the Suns came out sprinting, running their fast break even after a make by the Mavs and taking a 62-58 lead. Nash was the trigger in the first quarter with five assists, then had a handful of nifty layups in the second period, when he scored 12 points.

"It took us a little bit at the begining to adjust from the pace of the last series to this series," Nowitzki said. "They had a couple of leakouts early and just had layups on us, but I think that when that first quarter was out of the way, I think we were fine."

Harris, who disappeared at the end of the conference semifinals, re-emerged in the third quarter and gave the Mavericks the energy boost they needed. He scored 13 points, including several driving layups through Phoenix's porous defense.

"He was great," Nash said. "For stretches he kept them in it. He was terrific from the perimeter and he got to the basket. He had a great game."

A three-point play by Daniels capped a 13-0 spurt that gave Dallas a 100-91 lead early in the fourth quarter.