David Ferrer and Grigor Dimitrov were among the men's seeds through to the third round of the US Open on Friday.
Fourth seed Ferrer was handed a walkover when opponent Bernard Tomic pulled out with hip problems.
The Australian had surgery on both hips after the Australian Open and did not want to take a risk when he felt pain in the left one following his first-round win over Dustin Brown.
Tomic, who has also been struggling with flu, said: "I don't want to muck around with that area. For sure something is there. I've got to get it checked. It's not good right now. It's painful.
"I can't afford to get on court and play against David and cause much more pain to myself because I'm going to have to stay with him the whole match.
"I can potentially make it 10 times worse. For me it's the best thing not to go on court today. It's a very difficult decision for me, but I have to do this."
It has been a season of wild extremes for Tomic, who played the quickest recorded match in ATP Tour history on his comeback from surgery when he lost to Jarkko Nieminen in 28 minutes in Miami.
He subsequently dropped out of the top 100 after a torrid run only to win the second title of his career in Bogota in July.
In other action, seventh seed Dimitrov is through to the third round without dropping a set so far after a 6-1 6-2 6-2 victory over Dudi Sela.
Marin Cilic has also been impressive so far, and the 14th seed's 7-6 (7/2) 6-2 6-4 victory over Ilya Marchenko set up a third-round clash with 18th seed Kevin Anderson, who came from a set down to beat Jerzy Janowicz.
Gael Monfils, the 20th seed, avoided his customary drama in a 7-5 6-3 6-2 win over Alejandro Gonzalez, while 19th seed Feliciano Lopez won in four sets against Tatsuma Ito.
Monfils will take on fellow Frenchman Richard Gasquet in the third round after the 12th seed saw off Italian Paolo Lorenzi in straight sets.
But 11th seed Ernests Gulbis surrendered a two-set lead to lose 4-6 3-6 6-4 6-3 6-3 to training partner Dominic Thiem, who shares the same coach.