The biggest challenge facing the field at the U.S. Senior Open is to beat the heat and two timeless wonders who continue to find their way to the top of the leaderboard at major championships.

Sweating out each could prove equally problematic.

With a triple-digit temperature forecast Thursday and Friday in California's capital city, the toughest test on the Champions Tour has some added sizzle.

Bust out the sunscreen and sunglasses. Load up on water and soak up the shade. Between the competition and the conditions, this U.S. Senior Open should be a scorcher.

"We old guys like heat. We don't like cold. We like it hot. Keeps our bones and everything loose," quipped 65-year-old Tom Watson, an eight-time major champion on the PGA Tour. "You get it below 80 degrees, and we start putting our cashmeres on."

Navigating through the 72-hole grind of the national championship for seniors is always difficult. Doing it in 100-plus degree heat against a pair of past champions consistently wearing down opponents is another matter.

Besides the thermometer, all eyes will be on defending champion Colin Montgomerie and 2010 U.S. Senior Open winner Bernhard Langer at Del Paso Country Club, a green oasis in rain-deprived Northern California (the club says its water is drawn from an old, private well).

Montgomerie and Langer have combined to win six of the previous seven senior major championships. They're the leading money-getters on tour and the favorites to finish on top again.

"They seem to just continue to play the same all the time. They both are in contention on a weekly basis," said 50-year-old Lee Janzen, one of the youngest players on the Champions Tour.

The 52-year-old Montgomerie, who finished in a tie for 64th at the U.S. Open at Chambers Bay on Sunday, captured his second consecutive Senior PGA Championship this year. The Scot beat Gene Sauers in a three-hole aggregate playoff to win last year's U.S. Senior Open in scorching heat at Oak Tree in Oklahoma.

The 57-year-old Langer won his second straight Senior Players Championship this year. The German has won five senior majors, tied for fifth most. Jack Nicklaus is the record-holder with eight.

Langer and Montgomerie, friends and former Ryder Cup partners, are grouped with amateur Patrick Tallen in the first two rounds. They're scheduled to tee off in the afternoon Thursday, when the forecast high is 104 degrees.

"I'm not a big fan of heat," said Langer, a two-time Masters champion who lives in hot and humid South Florida now. "I'd rather put a sweater on and play in the 60s and 70s."

While Montgomery has won three senior majors the past two years, Langer has been the barometer on the Champions Tour since joining in 2007.

His six-stroke victory at the Senior Players Championship two weeks ago at Belmont Country Club was his 24th win on the Champions Tour. He has been the money leader six of the last seven years and is second on the money list this season behind Montgomerie, who has played one more tournament.

"It will be fun. It is fun playing with him. You know exactly where you are, where you stand the first two days," Montgomerie said.

The challengers for Langer and Montgomerie are vast and varied.

There are 28 past USGA champions, 27 amateurs and 84 sectional qualifiers among the field of 156. That includes three-time U.S. Open winner Hale Irwin and players such as Miguel Angel Jimenez and Kenny Perry who have contended in majors on the PGA Tour in recent years. It does not include Fred Couples; the 1992 Masters champion withdrew because of a back injury.

The nearly 7,000-yard, par-70 course has some unleveled lies but is about as straightforward as California courses come. The trick is to stay out of the long, lush rough and control the speed on the fast fairways and fickle greens — all of which will be tougher as the temperature rises and the competition heats up.

"Just stay in the shade as much as possible because it will be quite a while out there," Montgomerie said. "The course is playing difficult. It could take up to five hours to play, unfortunately, and that's a long time to have the concentration out there that one needs to win this type of championship."