Roger Federer and Rafael Nadal were challenged but claimed second-round wins at the US Open, while Grigor Dimitrov suffered a shock loss.
Favourites at the year's final grand slam, Federer and Nadal were pushed by Mikhail Youzhny and Taro Daniel respectively in New York.
But they survived to remain on track for a blockbuster semi-final meeting at Flushing Meadows.
However, Dimitrov - among those considered contenders - was stunned by Russian teenager Andrey Rublev.
Meanwhile, the likes of Dominic Thiem, Juan Martin del Potro and Gael Monfils advanced.
FEDERER, NADAL BATTLE THROUGH
Federer made it 17 wins from as many matches against Youzhny, but only just.
The Swiss great eventually secured a 6-1 6-7 (3-7) 4-6 6-4 6-2 victory over the ailing Russian, marking the first time in his illustrious career that Federer has been pushed to five sets in the opening two rounds of a major.
World number one Nadal was similarly tested, falling a set and break down against Japan's Daniel.
But the Spaniard steadied to close out a 4-6 6-3 6-2 6-2 victory over Daniel, who started to fatigue after his five-setter a day earlier.
DIMITROV DUMPED OUT
Coming off a title win in Cincinnati, it was expected Dimitrov - the seventh seed - may be able to challenge in New York.
But the Bulgarian made a second-round exit, losing to 19-year-old Rublev 7-5 7-6 (7-3) 6-3.
It continued a poor record for Dimitrov at the US Open, where he has never been beyond the fourth round.
Another seed making an exit was Tomas Berdych, who went down to Alexandr Dolgopolov 3-6 6-1 7-6 (7-5) 6-2.
THIEM, DEL POTRO CONTINUE RUNS
The sixth-seeded Thiem came through a tough second-round clash with Taylor Fritz, beating the American teenager 6-4 6-4 4-6 7-5.
Del Potro, the 2009 champion, was far more comfortable, claiming a 6-2 6-3 7-6 (7-3) win over Adrian Menendez-Maceiras.
Also progressing were Monfils and David Goffin, who were both pushed to five sets, and Feliciano Lopez, the Spaniard setting up a clash against Federer in a battle in which he holds a 0-12 record.