By Tom Pilcher

LONDON (Reuters) - Fifth seed Robin Soderling lost 6-1 6-4 7-5 to 18-year-old Australian qualifier Bernard Tomic on Saturday in the biggest upset so far in the men's singles at Wimbledon.

The youngest player in the men's draw, poised to end Lleyton Hewitt's 11-year reign as Australian number one, produced a brilliant all-round display to stun the ailing Swede, who fought back from two sets down to beat Hewitt in the last round.

"Most definitely the best achievement that I've done in my career so far and I'm really happy," the Australian qualifier told reporters after a standing ovation on Court One.

"I'll always remember this is the first time I've really done well at a grand slam," added Tomic, who came from two sets down to beat Igor Andreev in the second round having knocked out number 29 seed Nikolay Davydenko in the first.

The world number 158 showed no signs of fatigue from that match, however, and raced through the first set in just 17 minutes, before mixing rasping groundstrokes with delicate volleys and lobs to seal victory inside two hours.

Soderling, in contrast, appeared off the boil from the beginning, regularly draping his towel around his neck during changes of end and taking deep breaths as he ambled around the baseline between points.

"I felt weak. I felt a little bit dizzy. It just wasn't a good day," the 26-year-old told reporters.

"Yesterday and today I've been a little bit low on energy. I had some stomach problems."

Tomic will play 2002 semi-finalist Xavier Malisse of Belgium in the last 16.

"He's a dangerous player. He can definitely play good on grass," said Tomic after exchanging a few words with Malisse in the hallway outside.

"I know this is going to be the toughest match so far I will play against Tomic just because he has nothing to lose," said the 30-year-old Malisse.

(Editing by Ed Osmond)