AMA Motocross leader Mike Alessi injured.

AMA Motocross Championship leader Mike Alessi has undergone knee surgery, just days after a perfect 1-1 sweep of round three at Freestone County Raceway in Wortham, Texas.

The victory was Alessi's second of the season and pushed the Makita Suzuki rider 31 points clear of team-mate and double AMA Supercross champion Chad Reed.

However, Alessi's 2009 title hopes suffered what appears to be a mortal blow when he broke his kneecap in a practice accident on Monday. The American underwent surgery the following day.

"There was no other damage to the knee beyond the broken kneecap," said team manager Roger DeCoster. "The outlook is better than expected as we thought he was out for the year. It appears now that there's a good chance he will be able to get back out there racing before the end of this season."

Reed sits 17 points clear of third placed Josh Grant.

Round one winner Ryan Villopoto has also suffered an injury.