Mo Farah collected a fourth global championship gold with his 10,000m victory at the World Championships in Moscow on Saturday and will start the defence of his 5,000m title on Tuesday as he seeks the distance double at the same meeting.
A second success would see the Briton, who did the double at last year's Olympics, join Ethiopia's Kenenisa Bekele as the only men to win double gold in the distance events at both Olympics and Worlds.
BBC Sport athletics experts Steve Cram and Paula Radcliffe analyse the 30-year-old's performance in the Russian capital and assess his chances of matching Bekele's feat.
Former 1500m world champion Steve Cram
"With three laps remaining I thought we were in for a procession, but it didn't quite happen like that because of Ethiopian Ibrahim Jeilan's late push.
Mo's last lap [of 54.49 seconds], considering everything he's achieved, looked tougher than might have been expected, but winning gold medals isn't meant to be easy. It's supposed to be difficult.
It was good that Jeilan, the gold medallist from Daegu two years ago, made Mo work for it and, importantly, it wasn't a burn-out race. It wasn't particularly quick, which will help Mo in the 5,000m later on in the week. He'll come back strong.
It was a brilliant performance. Mo has now won another 10,000m title, his fourth gold medal at a global championship, and that's incredibly impressive. He's on top of the world.
An athlete can only compete against those running against him and the others aren't good enough to beat Mo.
If, one day, Mo breaks world records then we can deem him faster than everyone else who has gone before him but, for the moment, he's a championship runner and winning medals is what counts. It is for the gold medals he'll be remembered.
Discussing who is the greatest of the greats is something to be done down the pub - and Mo hasn't finished his career yet so it's not an argument we can have. Until he's retired, until we can stack up all his achievements, until we know how fast he's run, only then will we know his place on the all-time greats list - or we'll argue about it anyway."
Paula Radcliffe, winner of the 2005 world marathon title
"I think Mo can be spoken of alongside the likes of Haile Gebrselassie and Bekele. He's run three seconds quicker than Gebrselassie over 1500m which means Mo's range over the shorter distances is better.
He hasn't yet moved into the marathon, but when he does, having a runner with that phenomenal range, from the 1500m to the marathon, is not something we've seen before.
Mo's achieved everything he wanted to achieve, certainly over 10,000m. That was the one he wanted because of what happened in Daegu and to get the better of Jeilan will mean a lot to him. He was very frustrated, mad even, that he was beaten two years ago.
He had to work hard for the gold but hopefully he'll recover for the 5,000m. I'm confident he had that little bit extra in reserve had he needed it for that race and, anyway, winning helps you recover quicker.
It seemed a hard race. They were digging in and looked as if they were running faster than they actually were and that's because of the work they had done. They weren't necessarily running quickly in the early stages but the pace was often changing.
The conditions were stifling too. It was humid and Mo had to look after himself out there, keep his elbows sharp, because he was being targeted - a clip of the heels here, a push there - and that is draining too because you have to think all the time.
There were parts of that race where he didn't look great, whether that was because of the pressure he and others had put on him, I don't know.
But it's always nice to get that first race out of the way, and for it to go well. That will also help him recover quicker. Let's just say, it will be easier for Mo to bounce back for the 5,000m than his training partner Galen Rupp because finishing fourth is not nice."