NAIROBI, April 10 (Xinhua) -- Kenyan world marathon champion Geoffrey Kirui and silver medallist Edna Kiplagat jetted out to the USA on Tuesday beaming with confidence ahead of their titles defences in Sunday's Boston marathon.

Kirui says he's determined to defend his title and boost his credentials as he looks to clinch the World Marathon Majors crown later in the season.

Kirui believes he will face stiff competition from Galen Rupp, the first American in 15 years to win the Chicago Marathon in 2017.

"It will boil down to fitness. Rupp is a strong candidate to win, but I go into competitions knowing that it is do or die. I want to win and retain my title in Boston," said Kirui.

Alongside the fitness, Kirui noted that the weather, water and injuries to athletes will play a part as everyone lines up eyeing the title.

American Alberto Salazar, Rupp's coach and former Boston champ also believes the men's marathon will be a two-horse race pitting his athlete against Kirui.

"Galen will definitely be much harder to beat than last year, regardless of how the race plays out," said Salazar. "But Kirui or the others may also be in better shape than last year, so it's impossible to predict."

On the other hand, Kiplagat, 39, knows she's in the twilight of her career and will want to exploit every available chance to add another medal to her trophy case.

"I only enter a race when I feel 100 percent fit. I have done my training well and gauged myself. But a marathon is always an adventure and you have no idea what to expect from your body until you're in the middle of the race. I will focus on my strength," Kiplagat said.

Kiplagat had a successful 2017, running three marathons. She won in Boston, was second at the World Championships in London and came fourth in the New York marathon.

"After racing all around the world, it's special for me to return to Boston. I look forward to the challenge, once again racing against the world's best athletes," said Kiplagat.