As the women's heptathlon kicks off, two of Britain's most beloved track and field athletes go head-to-head for gold.
Jessica Ennis-Hill was the poster girl of London 2012, but could 23-year-old Katarina Johnson-Thompson be another legend in the making?
Two double-barrelled surnames and two of the brightest British medal hopes, but whose corner are you in?
Here is a five-step guide to help you decide.
Both Brits are massively talented, and some of the personal bests they have set in the seven heptathlon events are astonishing.
In the 200 metres, Kat has edged Jess by 0.04 seconds with a brilliant 22.79 time, and in the long jump she has beaten the Olympic champion by a foot (29cm) with 6.92m.
To put that in perspective, any jump over 6.48m is worth over 1,000 points in the seven-part heptathlon, and Jess's winning score in London 2012 was 6,955. If Kat pulls one of those out the bag she could be travelling up the table quite sharply.
Jess, meanwhile, is the heptathlon record holder for the 100m hurdles with a time of 12.54 seconds, which compares to a best of 13.37s of her compatriot. She is on top in the shot put and javelin too, with 14.67m and 48.33m throws respectively.
But the pair are almost a dead heat in the high jump, with 1.95m as Jess's best while Kat's is 1.97m. And there is only 0.2s between their 800m times, with Katarina just in front, so that too could be very tight.
As Real Madrid fans with Yorkshire accents will tell you, winning is everything when it comes to choosing sides. Betting site Paddy Power has Jess as the clear favourite at odds of 5/4 (or £12.50 back from your tenner). That's almost a certain gold for GB in the medal table, right?
Kat is only 2/1, but who doesn't like a plucky underdog?
As current Olympic champion, Jess, 30, has done this all before so getting in the right mindset will surely be a breeze? She told the Daily Telegraph she felt more stressed in 2012 than ever before but is now "back to the top" of her sport.
Meanwhile, Kat has not won a senior heptathlon event. She did compete at London 2012, finishing 15th aged just 19, and is the European indoor champion in the pentathlon.
Clearly both of them train. A lot. They've been taking pictures and videos of themselves putting in the hard graft in the past week and have posted them on social media. Nothing says prepared like an Instagram...
Those abs, though.
5. Twitter wisdom
Can these super-fans on social media persuade you either way?
Some are staunchly enamoured with Jess.
While others think Kat deserves a break.
Whatever happens, we're proud of both of you and feel lucky to have you in Team GB. Good luck!