Lleyton Hewitt described ex-lover Kim Clijsters as one of tennis' greatest players, one who should have won more Grand Slam titles than she did.
Clijsters, a four-time major winner who retired after her loss in the second round at the US Open this week, was once engaged to Hewitt, a two-time Grand Slam title winner and fellow former world number one.
But they split in October 2004, just four months before they were due to marry following a five-year relationship when they became the sport's most high-profile couple.
"She has been a great player, you know, one of the best in the women's game," said Hewitt, who now has three children with wife, Australian actress Bec Cartwright.
Clijsters has a daughter Jada with basketball-playing husband Brian Lynch.
"She should have won a lot more Slams probably than even the four she won. She easily could have won at Wimbledon. I think her game suited Wimbledon The French Open she came close a couple of times.
"Yeah, she has been one of the greats."
Hewitt turned in a trademark warrior effort on Friday to make the third round of the US Open, winning a four and a half hour marathon 3-6, 7-6 (7/5), 6-7 (5/7), 7-5, 6-4 against Luxembourg's Gilles Muller.
It was his 49th career five-set match, 31 of which he has won.
The 31-year-old Australian, who was the champion in New York in 2001 and runner-up in 2004, again defied the foot problems which have restricted him to 11 events this year.
But he still needed attention after damaging toenails, which left him with blood pouring into his shoes.
"I don't know if I'd call it an injury. It was more just the toenails were giving me some issues out there," said Hewitt, who underwent radical surgery in February this year when he had bone cut from his big toe and two screws and a metal plate permanently locked in.
"Just filling the sock up with a bit of blood. Just needed to get them taped and a little bit of padding to relieve it a little bit. Just got it sorted out."
The 31-year-old Hewitt, whose ranking has slipped to 125 in the world after his foot and groin problems kept him off the tour, has needed wildcards to enter all four Grand Slam events in 2012.
Despite his injury problems, the battling Australian said he has no intention of following either Clijsters or Andy Roddick, a fellow former world number one and 2003 US Open champion, into retirement.
"Andy obviously just felt like the time is right. Everyone is different. For me, I feel like I've done all the hard yards coming back from surgery. I sort of want to go out on my terms a little bit more," said Hewitt.
In Friday's match, Muller fired 35 aces and 95 winners, but also committed 69 unforced errors in a rollercoaster match on Court 11 with Hewitt ending the encounter with 60 winners, 30 unforced errors an 18 aces.
Hewitt goes on to face Spanish fourth seed David Ferrer for a place in the last 16.