Glasgow - Celtic winger James Forrest celebrated his new contract with a goal as the Scottish champions beat St Mirren 2-0, while title rivals Rangers routed Ross County 4-0 on Wednesday.
Forrest grabbed Celtic's second goal after the interval at Parkhead, just hours after signing a one-year contract extension which ties him to the club until 2023.
The 28-year-old came closest to breaking the first-half deadlock as St Mirren defended in numbers and displayed good skill and composure to net in the 54th minute.
Mohamed Elyounoussi had opened the scoring five minutes earlier from close range as Celtic ensured they remain top on goal difference.
"He was brilliant. We gave him a new deal and I don't care how much we pay him, he is priceless to me," Celtic boss Neil Lennon said of Forrest.
"He is in his peak years, it's 10 goals already only a quarter of the way through the season. That's a great return for a wide player.
"What would he cost? A goalscoring winger who has been doing it for 10 years consistently, treble player of the year, international player of the year - start the bidding at £25 million.
"He's not going anyway, we have got him secured."
Rangers ripped Ross County apart at the Global Energy Stadium, but remain just behind Celtic in second place.
Colombia striker Alfredo Morelos got the visitors up and running in the 20th minute when he knocked in a pass from Sheyi Ojo for his 17th goal of the season.
Ryan Jack scored twice to put Rangers in control by the interval.
Morelos grabbed his second with a drive in the 72nd minute, but Steven Gerrard's side couldn't add the extra goals that would have knocked Celtic off the top.
"The key thing was we started the game a lot better. In the last two domestic games we cruised in with an arrogance that is not us," Gerrard said.
"We are not at the level where we can do that. I was delighted with our mentality. That's what we need every game."
Hibernian's Martin Boyle enjoyed a fairytale return from injury with a last-gasp equaliser in a 2-2 draw against Livingston.
Motherwell moved into third place with an 2-1 win against 10-man Kilmarnock, while Aberdeen were 1-0 winners at Hamilton and St Johnstone beat Hearts 1-0.