In 2001, he beat future pros John Isner and Jo-Wilfried Tsonga, split with Mathieu Montcourt, and lost to Brian Dabul, Dudi Sela, Bruno Echagaray (3 times), Philipp Petzschner, Marcos Baghdatis, Adrian Cruciat, and Rajeev Ram.

In March, he played in Italy, winning 3 of the 4 weeks while beating Oliver Marach twice, Ilija Bozoljac, and splitting with Florian Mayer, to improve his ranking to No.

On the Italian Futures circuit in May, he earned more points, beating Diego Hartfield twice, Santiago Ventura, and Édouard Roger-Vasselin to take his ranking to No.

His first match win on an ATP Circuit event was over Stefan Koubek in Kitzbühel, before going down to Juan Ignacio Chela in three sets, taking the first.

He won the Rome Challenger defeating Andreas Vinciguerra in the final, on the Thursday of that week Köllerer's mother died of cancer and "he dedicated the title win to her memory and that she would be certainly proud of me"[1] Despite a spirited and entertaining performance, Köllerer was defeated in four sets in the third round of the US Open by the #6 seed and eventual champion, Juan Martín del Potro.

Köllerer was accused of racism by Brazilian tennis player Júlio Silva, who filed charges against him after a match on the ATP Challenger Tour at Reggio Emilia, in Italy, in June 2010.