Roger Federer believes Mario Ancic was instrumental in turning him into a humble champion.

Ancic was only 18 and ranked No154 when he faced Federer on Centre Court in 2002. The Croatian was not expected to survive against the previous year's quarter-finalist but won in straight sets.

The pair meet again tomorrow, with Ancic the last player to beat the world No1 on grass, and Federer said: "Back then, when guys maybe didn't have the proper technique or were new on Tour, I had a tendency not to give them the respect they deserved.

"I completely underestimated Ancic that day. I just thought I'd play a little bit of serve and volley.

"I expected him to stay back, but it was the opposite. I was shellshocked. It taught me never to underestimate any opponent."

Federer reached the last eight with a straight-sets win over Australian Lleyton Hewitt, the 2002 champion.

Ancic had a much harder job against Fernando Verdasco, taking almost four hours to beat the Spaniard, with the deciding set going to 13-11.