England's John Bickerton was second after a 66 for 137, with five players sharing third place on 138 - America's Scott Dunlap, South Africa's David Frost and Clinton Whitelaw, Paul McGinley of Ireland and Britain's Mark Roe.
Nick Faldo, who failed to make the cut in last week's Alfred Dunhill PGA in Johannesburg, added a fighting par 71 to his overnight 70 to remain in contention. Ernie Els, who won that tournament, is on the same mark, seven strokes off the lead.
Temperatures again rose to almost 40C and Roe offered some advice on how to prepare for the exhausting conditions. "Take your clubs and waterproofs to your local health club and sit in the sauna for five hours," he said.
Struver sounded a note of caution. "It's only half-time," he said. "A few tee shots strayed, and my irons were not too good. But I'm putting well, and if you putt well, you score well."
In Florida, Laura Davies scored six birdies in a five-under-par round of 67 to head the British challenge in the first round of the Naples LPGA Memorial at Pelican Strand, but she still trailed the leader Karrie Webb by four shots.
Webb led by two from a group of seven players that included the leading European, Sweden's Liselotte Neumann.
Helen Dobson, who missed out on a top 10 place last week when she was disqualified, and Scot Janice Moodie completed the round on 68, one shot ahead of her compatriot Catriona Matthew.
Fred Funk took a two-stroke lead in the Bob Hope Classic in Palm Springs, California. Funk followed his first-round 65 with a 68 to move to 11-under.
John Daly, partnered by the recently retired basketball player Michael Jordan, complied a 66, the day's lowest round. Another basketball player present was Charles Barkley.