BOSTON — Sisay Lemma scorched the first half of the Boston Marathon course on Monday, setting a record pace to build a lead of more than half of a mile.

Then the weather heated up, and the 34-year-old Ethiopian slowed down.

After running alone for most of the morning, Lemma held on down Boylston Street to finish in 2:06.17 — the 10th fastest time in the race’s 128-year history. Hellen Obiri defended her title, becoming the first woman to win back-to-back Boston Marathons since 2005.

“I decided that I wanted to start fast early,” said Lemma, who dropped to the pavement and rolled onto his back, smiling, after crossing the finish line. “I kept the pace and I won.”

Lemma, the 2021 London champion, arrived in Boston with the fastest time in the field — just the fourth person ever to break 2:02:00 when he won in Valencia last year. And he showed it on the course Monday, separating himself from the pack in Ashland and opening a lead of more than half of a mile.

Lemma ran the first half in 1:00:19 — 99 seconds faster than Geoffrey Mutai’s course record pace in 2011, when he finished in 2:03:02 — the fastest marathon in history to that point. Fellow Ethiopian Mohamed Esa closed the gap through the last few miles, finishing second by 41 seconds; two-time defending champion Evans Chebet was third.

CJ Albertson of Fresno was the top American men’s finisher in seventh place, timing 2:09.53. It was his second top 10 finish at Boston.

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Obiri’s Successful Title Defense

On a day when sunshine and temperatures rising into the mid-60s left the runners reaching for water — to drink, and to dump over their heads — Obiri ran with an unusually large lead pack of 15 through Brookline before breaking away in the final few miles.

Obiri also won New York last year, and is one of the favorites heading into the Paris Olympics. She said she told herself: “I’m not giving up. I’m not going to let this one go.”

Emma Bates of Boulder, Colorado, finished 12th — her second straight year as the top American.

Wheelchair Race Highlights

Switzerland’s Marcel Hug righted himself after crashing into a barrier when he took a turn too fast and still coasted to a course record in the men’s wheelchair race. It was his seventh Boston win and his 14th straight major marathon victory.

Hug already had a four-minute lead about 18 miles in when reached the landmark firehouse turn in Newton, where the course heads onto Commonwealth Avenue on its way to Heartbreak Hill. He spilled into the fence, flipping sideways onto his left wheel, but quickly restored himself.

“It was my fault,” Hug said. “I had too much weight, too much pressure from above to my steering, so I couldn’t steer.”

Hug finished in 1:15:33, winning by 5:04 and breaking his previous course record by 1:33. Britain’s Eden Rainbow-Cooper, 22, won the women’s wheelchair race in 1:35:11 for her first major marathon victory; she is the third-youngest woman to win the Boston wheelchair race.

100th Anniversary of Hopkinton as Starting Line

The otherwise sleepy New England town of Hopkinton celebrated its 100th anniversary as the starting line for the world’s oldest and most prestigious marathon, sending off a field of 17 former champions and nearly 30,000 other runners on its way. Near the finish on Boylston Street 26.2 miles (42.2-kilometers) away, officials observed the anniversary of the 2013 bombing that killed three and wounded hundreds more.

Sunny skies and minimal wind greeted the runners, with temperatures that rose into the 60s in late morning. As the field went through Natick, the fourth of eight cities and towns on the route, athletes splashed water on themselves to cool off.

“We couldn’t ask for a better day,” former New England Patriots tight end Rob Gronkowski, the grand marshal, said before climbing into an electric car that would carry him along the course. “The city of Boston always comes out to support, no matter the event. The weather is perfection, the energy is popping.”

The festivities began around 6 a.m., when race director Dave McGillivray sent about 30 Massachusetts National Guard members off. Lt. Col. Paula Reichert Karsten, one of the marchers, said she wanted to be part of a “quintessential Massachusetts event.”

The start line was painted to say “100 years in Hopkinton,” commemorating the 1924 move from Ashland to Hopkinton to conform to the official Olympic Marathon distance. The announcer welcomed the gathering crowds to the “sleepy little town of Hopkinton, 364 days of the year.”

“In Hopkinton, it’s probably the coolest thing about the town,” said Maggie Agosto, a 16-year-old resident who went to the start line with a friend to watch the race.

The annual race on Patriots’ Day, the state holiday that commemorates the start of the Revolutionary War, also fell on One Boston Day, when the city remembers the victims of the 2013 marathon bombings. At the finish line on Boylston Street, bagpipes accompanied Gov. Maura Healey, Boston Mayor Michelle Wu and members of the victims’ families as they laid a pair of wreaths at the sites of the explosions.