Roger Federer and Lleyton Hewitt struggled with stubborn opposition before advancing to the quarter-finals at the World Indoor Tournament on Thursday.

World number one Federer let a commanding first-set lead slip against Romania's Andrei Pavel before he edged through 7-6, 7-5. His victory set up a showdown with Britain's Tim Henman, who trounced Czech qualifier Tomas Cakl 6-2, 6-4.

Hewitt did just enough to overcome former Australian Open winner Thomas Johansson 7-6, 7-6.

In the late match, Spanish second seed Juan Carlos Ferrero required a third-set tiebreak to get past John van Lottum 7-5, 5-7, 7-6 and will meet another Dutchman, Raemon Sluiter, in the last eight.

Australian Open champion Federer started brilliantly against Pavel in a quest to extend his unbeaten run to a career-best 16 matches in ATP sanctioned events.

He frustrated Pavel with his array of penetrating and angled winners to race into a 5-2 lead in the opening set.

However, he suddenly seemed to lose interest and allowed Pavel to stage a fightback before regaining his focus to secure the tiebreak 7-2.

"I made life tough for myself. I gave him a chance and he took it and that made the match much longer than it should have been," said Federer.

Pavel continued to trouble Federer in the second. The top seed eventually broke the deadlock in the 11th game before sealing victory in the next game with a volley.

Britain's Henman will be looking to extend his 5-1 head-to-head record against Federer on Friday.

"I don't know why I've got such a bad record against Henman but I do," said Federer. "I really hope I can take my chances this time as I've lost some close matches against him."

TOUGHEST CHALLENGE

A losing finalist here on three previous occasions, Henman beat Federer at the Paris Masters quarter-finals in October -- the Swiss player's last defeat in an ATP event.

"(Beating Federer) is probably the toughest challenge in tennis right now because he hasn't lost any matches for a considerable amount of time," said fifth seed Henman, who was barely tested by the 213th-ranked Cakl.

"You look at the events he has won and they don't get any bigger than winning the Masters Cup (in November) and a Grand Slam but it's always fun to play Roger."

Stepping up his bid to end an 11-month title drought, Hewitt needed one hour and 43 minutes to tame Sweden's Johansson.

"I expected a tough match and that's what I got," the sixth seed said courtside with sweat dripping from his face.

Johansson, competing in only his fourth top-flight ATP tournament since missing the entire 2003 season through injury, matched Hewitt shot for shot in the first set as the players exchanged four breaks before the tiebreak.

Hewitt eventually bagged the set 8-6 in the tiebreak with a barrage of fierce groundstrokes.

The Australian came unstuck in the second set, however, as Johansson turned up the heat at the Ahoy arena with a series of unreturnable serves and angled winners.

Hewitt was broken in the fifth game and the match looked to be heading towards a deciding set as Johansson served to level matters.

Prowling the baseline, Hewitt rediscovered his touch in the nick of time to grab the break back.

He ran away with the tiebreak and sealed it 7-3 after the 276th-ranked Johansson sailed the ball long.