Englishman King, winner of the season-opening Scottish International Open and a quarter-finalist in Swansea for the past two years, led in both sets.  

But Ross, the former Irish international who won the Scottish outdoor national title last year, recovered from 4-1 down in the first set and 2-0 behind in the second to clinch a shock 7-5, 8-5 victory.

The 37-year-old from Paisley will now face England's fourth seed Mark Royal for a place in his first semi-final since 2007.

Royal, the world No3 from Stowmarket in Suffolk, faced New Zealand qualifier Peter Wilson and was behind the 39-year-old for most of the opening set.  But a double in the final end tied the set for the Englishman – and 2008 champion Royal went on to lead throughout the second to earn a 6-6, 8-7 win.

There will be Welsh interest in the last eight thanks to 2006 champion Jason Greenslade.  The No7 seed from Pontypridd faced the game's fastest-rising star Stewart Anderson, the 25-year-old Scot from Ayrshire who is a qualifier in the event but already certain of a world top 16 place next season.

Greenslade took the opening set by a three-shot margin.  But Anderson, now based in Pontnewynydd in Wales, battled back to draw level – then took the first end of the tie-break.

But Greenslade, spurred on by the partisan crowd, maintained his focus to snatch the final two ends to record a popular 7-4, 6-7, 2-1 victory.

England's world No2 Greg Harlow ended Neil Furman's brave run in the event when he beat the US qualifier 9-4, 9-4 to claim his first place in the quarter-finals since finishing as runner-up three years ago.

RESULTS:    Co-operative Funeralcare Welsh Grand Prix, Swansea, Wales

2nd round:
[14] Jonathan Ross (SCO) bt [9] Mervyn King (ENG) 7-5, 8-5
[3] Greg Harlow (ENG) bt [Q] Neil Furman (USA) 9-4, 9-4
[7] Jason Greenslade (WAL) bt [Q] Stewart Anderson (SCO) 7-4, 6-7, 2-1
[4] Mark Royal (ENG) bt [Q] Peter Wilson (NZL) 6-6, 8-7

Thursday (3 February) schedule:
10.00am, quarter-finals:
[1] Darren Burnett (SCO) v [6] Simon Skelton (ENG)    
1.00pm:
[13] David Gourlay (SCO) v [2] Paul Foster (SCO)    
[3] Greg Harlow (ENG) v [7] Jason Greenslade (WAL)    
[14] Jonathan Ross (SCO) v [4] Mark Royal (ENG)    

Friday (4 February) schedule:
10.00am, semi-finals:
Burnett or Skelton v Ross or Royal
Harlow or Greenslade v Gourlay or Foster
2.30pm
FINAL