FALMOUTH — Kenyan runners Stanley Biwott and Margaret Wangari took advantage of a steep downhill at the end of Sunday’s Falmouth Road Race, sprinting down Grand Avenue into Falmouth Heights to shake off the last of their challengers and win the men’s and women’s divisions of the 7-mile seaside run.

It was the second victory in a week for the pair, who each also won last week’s Beach to Beacon 10K.

Biwott, 26, edged defending champion Lucas Rotich in the last meter to cross the line in 31 minutes and 59 seconds, setting a 4:34.33 pace in thick humidity. Rotich clocked 32:01, making it the closest finish in race history. Biwott won the Beach to Beacon in 27:58.6 on a hot and humid day.

Wangari passed 2010 champion Wude Ayalew and Emily Chebet as she sprinted down the hill, claiming her third straight race, in 36:54. She was followed by Chebet (36:57), who passed Ayalew (36:58) in the last stride. Wangari, 26, also won the Bix 7 earlier in the summer.

“I’m a good sprinter because I have been running the 1,500,’’ said Wangari, who began her career in 2007 running middle distances.

Morning rainstorms were so heavy, first-year race director Dave McGillivray had to scramble to get huge puddles cleared from the road, delaying the start by 10 minutes. The rain had stopped by race time but many runners misjudged the severity of the humidity that quickly settled into the air. The medical tent was packed with overheated runners an hour after the winners crossed the line, and at least three were transported to the hospital.

“I think it was deceiving with the weather,’’ said Dr. Robert Davis, who was working in the medical tent. “The weather changed during the race. We’re actually quite busy.’’

Even Frank Shorter, a former champion here and an Olympic gold medalist in 1972, said the humidity was extraordinarily difficult.

“I couldn’t tell you how many Falmouths I’ve run,’’ Shorter said, “but I can tell you this one was the hardest.’’

Biwott, who plans to run the New York City Marathon, didn’t let humidity slow him. He said the up-and-down Falmouth course was a good training run, and smiled broadly.

Biwott also said he had no particular plan for Falmouth. “You never know who you can lead and what the race is going to be,’’ he said.

For the first half of the race, the lead pack was crowded, starting out with about 16 runners. By Mile 6, only the Kenyan trio of Biwott, Rotich, and Stephen Kipkosgei-Kibet remained.

“To me, I started pushing at 2 miles,’’ said Rotich, “and after 5 miles, I started to push, push, and push. Nobody else came. There were three at the last, and then I started to push again.’’

The very last push belonged to Biwott, as he stepped across the line a toe before Rotich.

“It’s a very good course, I like this place,’’ said Biwott.

Wangari said she continually tried to run away from the other women in the lead pack.

“I was trying to push and get my colleagues to stay away,’’ she said. “At 6 miles, there was one lady in front [Ayalew] and she pushed the pace. I followed her, and then I passed her when we were going down [the hill].’’

Wangari’s next stop is home in Kenya, but after such a smashing summer season, she wants to come back next year “to defend all my titles.’’

Krige Schabort of Georgia, 48, won his fourth wheelchair title (2011, ’09, ’08) in 23:52.16, and 19-year-old Jill Moore claimed the women’s title in 39:09 in her Falmout debut.

Of the 12,800 registered runners, 10,600 finished.