One of the best bike racers of all time jumped into an amateur race in Chicago and put on a clinic

Daniel McMahon/Business Insider

Two-time cyclocross world champion and Olympic cyclist Sven Nys awaiting the start of the Category 4/5 amateur race at Caldwell Woods, Chicago, part of the ChiCrossCup series, September 10, 2017.

CHICAGO - One of the best bike racers of all time, Belgium's Sven Nys, lit up the local cycling scene here on Sunday when he jumped into a beginners' cyclocross race at Caldwell Woods and ripped around the 1.5-mile course for a few laps before pulling out and letting the local riders battle it out for the finish.

Nys rolled up to the back of the packed field of Category 4/5 riders, the entry level for racing bikes in the United States. Nys had not announced he was racing the ChiCrossCup and surprised everyone when he lined up in race gear with number pinned for the last race of the day.

"This is like LeBron showing up to a 4th grade pickup game," Wall Street Journal sportswriter Jason Gay tweeted. "But it's awesome."

Nys put on a show, wowing spectators as he flew through 100 or so riders to the front of the field before the first of four laps had even ended. When he got up to the leader, he sat in behind the local, in second wheel, before a friendly back-and-forth rivalry ensued.

Along the way the two-time Olympian displayed the bike-handling skills that helped him win two world titles and some 140 races. A video on Twitter of Nys bunny-hopping a barrier and then riding up the side of a set of stairs, coming up just short, then repeating it successfully, went viral on cycling social media:

At one point he helped himself to a mid-race beer:

Nys was in Chicago with fellow Belgian 'cross star Sven Vanthourenhout to put on skills clinic hosted by local team xXx Racing to benefit the Pieter Ombregt Scholarship Fund. Nys, 41, retired last year from cyclocross, a sport he dominated for the better part of two decades.

In addition to his stellar cyclocross career, Nys raced mountain bikes and competed in the Olympics twice in that discipline. Along with Dutch star Marianne Vos, Nys is considered the best cyclocross rider of all time.

Here's Nys during his clinic showing locals how to bunny-hop a set of barriers.

And here he is dismounting his bike at speed and jumping a barrier:

Here he rides alongside local riders as they practice their starts:

Nys was scheduled to travel next to Iowa City for the Jingle Cross World Cup race, September 17, with his Telenet-Fidea cyclocross team, which he owns and manages. On September 25, Nys and Co. will be in Wisconsin for the Waterloo World Cup, hosted by Trek Bicycle. Nys is a Trek brand ambassador.

What's cyclocross?

Cyclocross combines elements of mountain biking and road cycling but is really its own sport. Riders pedal but also run as they hop on and off their bikes, jumping barriers and going up and down short climbs and off-camber sections, on technically challenging courses that can feature a mix of pavement, gravel, grass, sand, dirt, and mud.

Races are held rain or shine, with the season starting around September and finishing around January. The bikes look like road bikes but have knobby tires and other features that make them suitable for off-road riding. Courses are short, about a mile long, and riders complete several laps with the winner being the first person across the line. Amateurs typically race for up to 45 minutes with the pros going a full hour or even longer.

It's a demanding sport that requires a high level of fitness, power, and skill.

Cyclocross is most popular in Belgium and northern Europe and is the fastest-growing discipline in US cycling. Old than the Tour de France, cyclocross started as a way for road cyclists to keep fit in the winter, but it's now very much its own sport, and it has seen explosive growth in the past decade, especially in the US.

Some would like to see cyclocross become an Olympic sport.

See more photos from the race and clinic below.