Rio Olympics 2016: Jessica Ennis-Hill leads heptathlon, Katarina Johnson-Thompson fourth

Great Britain's Jessica Ennis-Hill is in gold-medal position, with compatriot Katarina Johnson-Thompson fourth, after a see-saw first day of the heptathlon at the Olympic Games in Rio.

After four of the seven events, defending champion Ennis-Hill, 30, leads by 72 points from Belgian Nafi Thiam. Akela Jones, from Barbados, is a further 21 points back.

Johnson-Thompson, 23, had led going into the evening session, only for a disastrous shot put of 11.68m to drop her back.

• GB's Rutherford scrapes into long jump final

Thiam's emergence adds a new dimension to the battle that began as a showdown between the two Britons, with world silver medallist Brianne Thiesen Eaton down in sixth, 186 points off the lead.

Four years ago, Ennis-Hill was 184 points ahead going into the second day, while she held an overnight lead of 80 over Johnson-Thompson at the World Championships a year ago.

Here in the Estadio Olimpico,she moved back into first place with a time of 23.49 seconds in the day's final event, the 200m.

It leaves her in promising shape as she looks to retain the title she won in such memorable style in London four years ago.

"It's been a mixed day," she said. "l was really happy with my hurdles and high jump. I was obviously disappointed with my shot put, that was frustrating.

"I was a bit down on the 200m but generally happy. I need to rest up and come back strong tomorrow. I'm trying to enjoy it but it's very stressful."

Johnson-Thompson will expect to eat into Ennis-Hill's advantage in the long jump, Saturday's first event at 15:45 BST, but Ennis-Hill has the better javelin.

The Britons are evenly matched over the 800m, while Thiam is expected to drop back in that final event.

Ennis-Hill had run 12.84 seconds in winning her 100m hurdles heat to give herself an early edge and then cleared 1.89m in the high jump.

That was her best height since the 2012 Olympic trials, and meant she had made her highest scoring start to a heptathlon since she gave birth to son Reggie two years ago.

But Johnson-Thompson then went clear at a new British record of 1.98m, having clocked 13.48 in the hurdles, to storm into the lead.

When Thiam also cleared 1.98m - a height no heptathlete has ever matched - that put the 21-year-old Belgian into second.

Johnson-Thompson went into the evening session on 2,264 points, an advantage of 12 over Thiam and 22 on Ennis-Hill, only for that sub-standard shot to drop her back to sixth.

The event has always been a weakness, but this was a metre and a half down on her personal best. Thiam's third-round 14.91m pushed her briefly into top spot, with Ennis-Hill's 13.86 enough for second overall.

But Thiam is a better jumper and thrower than runner and her 25.10 for third in the first 200m heat cost her as she chases her first major international medal.

"I was thrilled with my high jump," said Johnson-Thompson. "The 200m is one of my strongest events and I really wanted to drop under 23 seconds.

"They keep you out in the cold a long time, but that's no excuse, it's the same for everyone, but maybe that's why the times are down."

Mixed fortune for GB athletes

In the 800m, Britain's Michael Rimmer, at his third Olympics yet seeking to make his first final, qualified well from a heat containing reigning champion and world record holder David Rudisha.

There was disappointment for team-mate Elliot Giles, seventh in his heat, but both Laura Muir and Laura Weightman are into the semi-finals of the 1500m.

Matt Hudson-Smith qualified comfortably from his 400m heat yet there was no progression for Martyn Rooney, left distraught by a poor run for fifth in the fourth heat in 45.60 seconds.

"That was awful," he said. "It was embarrassing. You need to be confident, but maybe I was just too cocky."

Sophie Hitchon is into the hammer final after an impressive 70.37m in qualifying, while Desiree Henry and Asha Philip are safely into the next round of the 100m. Daryll Neita, 19, finished just outside the fastest loser spots.