Kevin Braniff scored direct from a free-kick to give Glenavon victory over Linfield at Windsor Park.
The former Portadown star lifted the set piece over the defensive wall for the 41st minute winner after Jimmy Callacher's hand ball offence.
Glenavon skipper Kris Lindsay was sent-off on 70 minutes when a foul on Ivan Sproule earned him a second yellow card.
It was Linfield's first league defeat since the opening day of the season.
Glenavon's win leaves the Irish Cup holders tied with Mid-Ulster neighbours Portadown and Ballymena United at the top of the Irish Premiership table.
The Ports, who beat Ballinamallard 3-0 at Shamrock Park, are the leaders on goal difference.
Glenavon started the match brightly, showing a positive attacking approach with Shane McCabe and Braniff pulling the strings in the middle of the field.
They got their reward through the accurate boot of summer signing Braniff four minutes before the interval.
Linfield showed more urgency in the second half but it was Glenavon who could have scored again when Andy McGrory shot wide.
Former Linfield defender Lindsay was dismissed after a sliding challenge halted Sproule's run down the right.
But Linfield failed to make the extra man count, with Alan Blayney making good saves from Linfield's Michael Carvill and Andrew Waterworth.
For Glenavon, Ciaran Caldwell saw a shot strike the crossbar.