The 29-year-old won European gold last year before going on to reach the semi-finals of London 2012, failing to advance to a final that saw Team GB's Dai Greene cross the line fourth.
But with reigning world champion Greene yet to start his season after undergoing double hernia surgery in March, Williams was left to battle with London 2012 team-mate Jack Green in Loughborough.
And while Green failed to finish – just as he had done in his London 2012 semi-final – Williams had no such problems as he set a new personal best time of 48.90 seconds with victory.
Williams has already booked his place at this year's World Championships in Moscow and next year's Commonwealth Games in Glasgow and believes he can go even quicker.
"I know I'll need to go quicker in Moscow to get a medal but I am happy with how things are going," said Williams. "I'm 29 and I’ve just run a personal best so it’s not bad.
"I wanted to beat Jack Green, he beat me at the (Olympic) trials last year but that gave me fuel over the winter.
"I had him outside me and I was gunning for him and I’m happy with the win.
"I've changed coaches for this season, moved back to training in Cardiff and it seems to be working so far.
"It's still only May, I know there is a long way to go, but I’m pleased.
"I am normally chasing the qualifying times but it's nice to have them out of the way for a change."
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- Dai Greene