Ben Spies was left with mixed emotions following the fourth World Superbike Championship round at Assen after seeing his outstanding victory in the opening race of the weekend undermined by an early crash in the second.

Spies' day started superbly, the American winning the race with a daring final lap move on Noriyuki Haga, one that came after he had seemingly slipped out of contention with only five laps of the race remaining.

However, while he admits he was ready to settle for third, the Yamaha rider became stronger as Haga and Leon Haslam began to fade, allowing him to launch a late race challenge.

Prevailing in an enjoyable series of overtaking moves with Haslam, Spies quickly caught Haga before outfoxing him on the run into the high-speed turn 15 kink on the final lap before crossing the line a 'spectacular' winner.

"In the race the bike didn't seem to have the same feeling it had in practice," he said. "I ran as hard as I could, but realised it might be a tough day for me when I got passed by Haga and Haslam.

"With about five laps to go the bike started coming around and I thought I might have a chance. Haslam and I went side by side through three turns and I was fortunate to get through. It was good clean racing.

"I got behind him on the last lap and just waited for an opportunity to present itself. I got good drive out of the fifth-gear right hander and snuck inside of him and that was it. It felt pretty spectacular to win like that."

Beating Haga in a straight fight around an iconic biking circuit, Spies was feeling confident of repeating his feat in the second race.

However, he lasted no more than a single lap after clipping the grass on the entry to the first turn and being sent into a high-side. With his bike catapulting over the barriers, Spies' injuries were limited to his pride, rather than his body.

"The bike felt very normal and I was able to get into the lead," he said. "On the second lap coming out of [turn] one I just barely touched the edge of the track on the grass and had a big high-side.

Nonetheless, Spies was feeling philosophical about the situation, saying 'that's just how it goes'.

"It was unfortunate because I felt like I was riding well and the bike was great. I mean I barely got the bike off the edge and that was the end of the race. That's just how it goes."