(CNN) -- Branden Grace stepped up his bid for back-to-back tournament wins by taking a four-shot lead at the halfway stage of the Volvo Golf Champions tournament at The Links at Fancourt Friday. 

The 23-year-old South African carded a seven-under-par 66 to set the pace on 12-under 134 after two rounds. 

Compatriot Thomas Aiken, who shot 70, and England's Lee Slattery, who raced up the field with a best-of-the-day 65, are his nearest challengers. 

European Ryder Cup captain Jose Maria Olazabal of Spain showed he is still a force at 45 with an impressive 68 to be alone on fourth on 139. 

Grace only received an invitation to the 35-strong champions-only field by winning last week's Joburg Open and has seized his opportunity in fine style. 

He was paired with first round leader Nicolas Colsaerts of Belgium, who crashed to a 76 to fall six shots off the pace. 

Home favorite Ernie Els, fellow three-time major winner Padraig Harrington of Ireland and reigning British Open champion Darren Clarke of Northern Ireland are eight shots off the blistering pace being set by Grace. 

Meanwhile, Colombia's Camilo Villegas and David Toms of the United Stages led after the first round of the PGA Tour's $5.6 million Humana Challenge in California. 

Villegas carded his nine-under 63 on the Nicklaus Private Course and Toms matched him at La Quinta Country Club. 

South Korean rooke Bae Sang-moon was in the chasing group on 64 with Ted Potter, Brandt Snedeker and Bob Estes. 

Toms played with star attraction Phil Mickelson, who struggled to a two-over 74 after a triple-bogey seven at the par-four 14th, on the toughest of the three courses used for the tournament. 
How old is Grace?
The 23-year-old South African