Credit: Chris Brunskill/Getty Images

Run, Pippa, run!

Pippa Middleton didn't so much sweat as glisten at a triathlon in Woodstock, England on Saturday.

Clad in a tank top, running shorts and sneakers, Princess Kate's little sister, 27, was radiant and strong as she ran among other competitors -- colliding with one handsome, topless mystery man! -- for Saturday's GE Blenheim Triathlon at Blenheim Palace. See more photos of Pippa running.

Kate, 29, meanwhile, was in more formal attire at Derby Day in Epsom, England -- regal in a cream-colored short skirt, high-heeled shoes and a light-brown hat as she joined a natty-looking Prince William, Prince Harry, Queen Elizabeth and other royals for a big horse race.

Even before her in-shape physique (including a talked-about derriere) made headlines worldwide at the royal wedding April 29, Pippa has long endeavored to stay healthy, active and in shape -- thanks to weekly pilates sessions plus swimming, tennis, skiing and, obviously, jogging.

Although "best friends," the Middleton sisters have always had a "healthy competition," royal watcher Katie Nicholl recently told Us Weekly. "Certainly when it comes to looks, like who is the slimmest or better looking, there is a little friendly rivalry," Nicholl said.

The siblings squabble over "weird things," one source added. "You'd see them fight over who had the worst headache or period pain!"