Then Groombridge (Action Moto Suzuki RM-Z450) wrapped up the premier MX1 class at the Auckland Motocross Championships at Tuakau, near Pukekohe, on Sunday.
He traded blows all day Sunday with Waitakere's Ethan Martens (Altherm JCR Yamaha Racing YZ450F), the two sharing wins in the first two MX1 races and lining up for the deciding third race level on points.
Martens led from the start but Groombridge dug deep, snatching the lead about halfway through and stretching out his advantage to win comfortably and claim overall victory for the day.
"Ethan qualified faster than me," Groombridge said. "I had dislocated my shoulder at a boot camp training session at Taupo last week and so I was just taking it easy, testing out how my shoulder would hold up.
"Fortunately, it felt okay. My shoulder felt weak but I got there in the end. I got a bit of arm pump in race one because I was nursing my shoulder.
"In the second race I was leading when I crashed just two laps from the finish. But I went on a charge and got back up and got past Ethan for the win."
If Groombridge maintains his winning momentum he will be among the favourites for the MX1 title when the New Zealand Motocross Championships start near Timaru in early February.