The Australian had looked to be struggling a bit for a few holes before dropping to one knee after hitting his tee shot at the 16th hole, where he closed out a 3 and 2 victory at Austin Country Club.
Day, a former champion, politely declined a television interview before heading to the on-site physio truck for treatment. While the extent of his injury was unknown, it was being speculated that Day might concede his second match, against Thailand’s Thongchai Jaidee, before returning to play Paul Casey in the final set of round-robin matches tomorrow.
Fresh from winning the Arnold Palmer Invitational in Orlando on Sunday, Day had overcome an early two-hole deficit to come out on top against McDowell, who is no slouch in head-to-head golf. The Northern Irishman clinched victory for Europe in the 2010 Ryder Cup at Celtic Manor before beating Jordan Spieth in the top singles in the biennial bout at Gleneagles four years later.
On a day when players and caddies were wearing black ribbons in tribute to the victims of the terrorist attacks in Brussels, the tenacious Jaidee pulled off a mild shock as he beat Casey by two holes.
Other early winners included Phil Mickleson, Sergio Garcia, Charl Schwartzel and Jason Dufner, the latter beating Scottish Open champion Rickie Fowler 2&1 in an all-American clash.
After his hard-earned one-hole success over Ryder Cup playing partner Lee Westwood – it involved just three halved holes – Garcia admitted: “It was very funny, up-and-down match and I was happy to get the win.”
Schwartzel, who recently chalked up his third triumph of the 2016 campaign, never trailed as he beat New Zealander Danny Lee on the last green. “At one stage I was playing really solid, and it sort of felt like I was in control of the game,” said Schwartzel. “But Danny made a really good comeback and I’m just happy I hit some good shots coming in.”
England’s Andy Sullivan also made a winning start, beating Austrian Bernd Wiesberger 3&2. World No 1 Spieth and defending champion Rory McIlroy were among the later starters, as was Russell Knox, who was up against Swede David Lingmerth.
Elsewhere, former Scottish Amateur champion Chris Robb is sitting just three shots off the lead in joint 12th after the opening round of the MENA Tour’s Royal Golf Mohammedia Open in Morocco. As Englisman Andrew Marshall set the pace, Robb carded a one-under 71, a shot better than two compatriots, defending champion Paul Doherty and Duncan Stewart.