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 US. 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.”

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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 @ChiCrossCup @sven_nys shows us how to bunny hop and take the stairs pic.twitter.com/b0UOwbCs1g — Peg Keiner (@PegKeiner) September 10, 2017

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.

“That was really fun,” Nys told Business Insider postrace. “It’s not all about racing. It’s also about having fun and letting the people see what cyclocross is all about. I have the technique, and I can show them somthing in a race. You hear the noise of the people – that’s what we need. Racing is fun.”

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.

Cyclocross has seen explosive growth in the US. The ChiCrossCup is an 11-race series.

Foto: source Daniel McMahon/Business Insider

Nys jumped into the beginners’ race unexpectedly and under the radar.

Foto: source Daniel McMahon/Business Insider

Spectators went wild when they realized Nys was racing.

Foto: source Daniel McMahon/Business Insider

After the race, the Svens Clinic began with Nys showing his stretch routine.

Foto: source Daniel McMahon/Business Insider

The clinic was very much hands on with both Svens showing proper form and technique.

Foto: source Daniel McMahon/Business Insider

One of the things that makes cyclocross unique is that riders carry their bikes and run with them. Nys showed how to carry one’s bike effectively.

Foto: source Daniel McMahon/Business Insider

Nys is an excellent instructor, showing and explaining all the basic skills.

Foto: source Daniel McMahon/Business Insider

Vanthourenhout, in Belgian kit, helped Nys teach the riders how to get started.

Foto: source Daniel McMahon/Business Insider

Nys also showed the local riders in Chicago how he bunny-hops barriers.

Foto: source Daniel McMahon/Business Insider

Master Nys was there to offer individual feedback on cyclocross technique.

Foto: source Daniel McMahon/Business Insider

Riders of all ages and abilities got to learn from one of the world’s best cyclocross riders.

Foto: source Daniel McMahon/Business Insider

Running up and down short, steep climbs differentiates cyclocross from other disciplines.

Foto: source Daniel McMahon/Business Insider

Nys showed how to approach and hop over barriers.

Foto: source Daniel McMahon/Business Insider

Vanthourenhout got up close when showing how to ride off-camber sections.

Foto: source Daniel McMahon/Business Insider

The recently retired Nys is now a brand ambassador for Waterloo, Wisconsin-based Trek Bicycles Corp. He rides a Trek Boone cyclocross bike.

Foto: source Daniel McMahon/Business Insider

Nys’ Boone has some custom touches.

Foto: source Daniel McMahon/Business Insider

Nys’ employer is hosting the Waterloo World Cup race in its backyard this September.

Foto: source Daniel McMahon/Business Insider

Nys rides Shimano’s electronic-shifting system, Dura-Ace Di2.

Foto: source Daniel McMahon/Business Insider

Nys rides Shimano SPD pedals.

Foto: source Daniel McMahon/Business Insider

These are Shimano disc brakes.

Foto: source Daniel McMahon/Business Insider

There’s plenty of grippy tread and mud clearance through the fork and frame.

Foto: source Daniel McMahon/Business Insider

Nys rides a carbon stem from Bontrager, a Trek brand.

Foto: source Daniel McMahon/Business Insider

There’s no confusion over who owns this Trek.

Foto: source Daniel McMahon/Business Insider

Vanthourenhout rode a Ridley CX bike. Note the wide knobby tires and disc brakes.

Foto: source Daniel McMahon/Business Insider

From what we could tell, the participants in the clinic learned a lot and were excited for their next race, having now learned valuable pro tips from the two Svens.

Foto: source Daniel McMahon/Business Insider

Foto: source DirtWireTV/YouTube