New Zealand's Bevan Docherty has headed off cycling legend Lance Armstrong in a thrilling finish to the Panama half ironman.

Two-time Olympic triathlon medallist Docherty ran down Armstrong this morning (NZT), using his renowned finishing legs to head off the American who was making his debut as a professional in the Ironman series.

Docherty's time of three hours 50 minutes 13 seconds was just 31sec ahead of Armstrong.

Australian Richie Cunningham rounded out the top three in the inaugural race with 3hr 52min 59sec.

It was Docherty's first half Ironman in 12 years.

"This is absolutely awesome," he said.

"That was one hard race. I wasn't too sure what to expect, but Lance really pushed me. I should probably sit down."

Armstrong appeared unhappy to have been passed by the Kiwi, walking straight past Docherty without congratulating him shortly after crossing the finish line.

The result will provide a boost for Docherty who is still to qualify for this year's London Olympics.

Armstrong impressed in the gruelling multisport event, raced in hot conditions.

"I need a challenge in my life," said the cancer survivor.

"I need some stuff to do. I like to train. I like to suffer a bit and today was little over the top, but it's great to be back."

The first time race attracted a deep pro field including defending Ironman 70.3 Texas champion Chris Lieto, Denmark's Rasmus Henning and Richie Cunningham among the big names.

The event began with a 1.9km swim in the Panama canal.

Former New Zealander Matt Reed, who now races for the United States, led the men out of the water with an 18min 49sec split. Docherty was third with an 18:54.

French pro Bertrand Billard took the lead from Reed early on the 90 km bike but it was no surprise that the seven time Tour de France winner moved into the top three before the end.

Lieto lead early on the 21 km run before Armstong took over.

However, Docherty slowly gained ground the take the lead in the final kilometre.

Canadian Angela Naeth won the women's race in 4:15.31 after emerging 16^th out of the water.

More than 600 athletes from around the world competed in the two loop run course, which is one of nearly 60 events in the global Ironman 70.3 Series.

Leading results:

Bevan Docherty (NZ) 3:50.13 1, Lance Armstrong (USA) 3:50.55 2, Richie Cunningham (Australia) 3:52.59 3, Rasmus Henning (Denmark) 3:53.38 4, Romain Guillaume (France) 3:54:44 5

- © Fairfax NZ News