Weekend rain caused a backlog of matches, and day-long showers Monday forced play to be pushed back until just after 6 p.m. Eastern time.
In a pair of matches that began Sunday night, Sampras got past No. 33 Greg Rusedski of Britain, 7-6 (7-4), 4-6, 7-6 (7-3), 3-6, 6-4, while Haas rallied past No. 29 Thomas Enqvist of Sweden, 6-4, 3-6, 3-6, 6-4, 6-4.
Sampras improved to 9-1 lifetime against Rusedski and won for the 21st time in his last 22 encounters against a lefthander.
Sampras, 31, gained the decisive break in the 10th game of the fifth set. He hit an inside-out forehand winner to get to 30-30, then hit a forehand winner down the line to get to match point. Rusedski's forehand down the line was just out, ending the duel between the big servers in three hours, 15 minutes.
"It was very exciting," said Sampras, who has reached the fourth round in each of his last 12 Open visits. "The people and the atmosphere out on Louis (Armstrong Stadium) was something that I
was enjoying. It made it a little bit sweeter with the win. Those are the moments that as you get a little bit older, you kind of cherish a little bit more."
The Open runnerup the last two years has not won a tournament since his record 13th Grand Slam at Wimbledon in 2000, but the former world No. 1 looked strong against a tough opponent, hitting 81 winners, including 17 aces, to 42 unforced errors.
"I feel pretty good," Sampras said. "I feel like it's a tough match. Playing Greg is very difficult. You don't get a ton of rhythm. It's kind of hit-and-miss tennis. You kind of hang in there against Greg."
Rusedski, who had an eight-match winning streak end, was less gracious.
"Well, I lost the match; he didn't win tonight," he said. "I mean, 5-3 serving for the first set (Sunday night), you know, double faults really hurt me that first set, especially when I had set point at 5-3 last night. After that, in the tiebreaker, you can't double fault twice against him. I played two horrendous tiebreakers. He's not playing great. I'd be surprised if he wins his next match against Haas. I mean, I lost the match rather than he won. I don't think he's playing (well). I don't really see it."
Sampras takes a 4-3 career edge into his fourth-round matchup with Haas, but the 24-year-old German has won the last three meetings, including last month at the Tennis Masters Series
event in Cincinnati.
"In Toronto, he served unbelievably well," Sampras said. "He served about 30 aces. I had some chances to maybe pull that match out. Tommy Haas has improved quite a bit the last couple of years. He's developed a big first serve. He backs it up with good groundies. He's moving well. He's a good player."
Haas survived his second five-set match of the tournament, and reached the fourth round at the Open for the third time in four years, equaling his best Open result. His best Grand Slam efforts have come at the Australian, where he appeared in the semifinals for the second time this year.
So far, Haas' sore right arm has held up well. He developed tendinitis in his forearm muscles two weeks ago on Long Island after overcompensating for shoulder tendinitis.
"Shoulder is hanging in actually quite well, which is great," said Haas. "So is my arm, even though it was kind of cold to play in the circumstances."
In a women's fourth-round match, Hantuchova upended Henin, 6-1, 3-6, 7-6 (7-4), to advance to a quarterfinal meeting with top seed Serena Williams.
"I'm just so happy to be in the quarterfinals," Hantuchova said. "I'm really looking forward to this match (with Serena). I played her in Wimbledon and I got a lot of experience from that (loss). I think I'm ready to play her."
Hantuchova was leading, 6-1, 1-2, on Sunday when, with rain falling softly, she slipped on the baseline. The 19-year-old Slovakian tumbled to the court, tearing ligaments in her right thumb and spraining her left ankle.
She took the court with her thumb and ankle taped, but removed the bandage on her hand after losing the second set. She iced the thumb after the match and is taking pain-killers.
"I didn't have the same feeling as I have without it," Hantuchova said. "I just thought it was going to be better. It still hurt, but I felt like that was a good decision."
Jennifer Capriati and Lindsay Davenport eased into the quarterfinals, but seventh-seeded Kim Clijsters was eliminated by No. 10 Amelie Mauresmo.
Capriati, the No. 2 seed, dispatched U.S. countrywoman Amy Frazier, 6-1, 6-3, after Davenport, the fourth-seeded former champion,
beat No. 13 Silvia Farina-Elia of Italy, 6-3, 6-1.
Capriati, the Australian Open winner, will face Mauresmo in the quarterfinals after the Frenchwoman posted a 4-6, 6-3, 7-5 win over her Belgian foe.
Mauresmo has taken her last two matchups with Capriati, winning this year at Wimbledon and Montreal, to even the head-to-head at 3-3.
In men's quarterfinal action, No. 1 seed Lleyton Hewitt of Australia continued defense of his title with a 6-4, 6-2, 7-5 victory over No. 14 Jiri Novak of the Czech Republic. Agassi, a two-time champion who is seeded sixth, cruised past fellow American Jan-Michael Gambill, 6-2, 6-3, 6-3.
Chile's Fernando Gonzalez, seeded 28th, upended No. 7 Juan Carlos Ferrero of Spain, 6-4, 6-4, 6-4, in a third-round encounter. Gonzalez will next square off with France's Arnaud Clement, who sidelined Argentina's Guillermo Coria, 2-6, 6-2, 7-5, 7-6 (7-5).
Former world No. 1 Gustavo Kuerten of Brazil also moved into the fourth round. He was leading, 6-1, 5-4, against Nicolas Massu before the Chilean retired with a right hamstring and groin injury.