After three serene rounds, Novak Djokovic was tested by Roberto Bautista Agut in the fourth round, and the world number one will expect another stiff examination when he plays Feliciano Lopez in the quarterfinals of the 2015 US Open on Tuesday.
Djokovic had made everything look easy in the first three rounds, and, therefore, the struggles against Agut, particularly in the second set, which he lost, came as a bit of a surprise.
However, Djokovic, as he always seems to do, found a way past that mire he found himself in after set two, to close out the match in four sets.
"I started allowing him to control the rallies," Djokovic said of that second set he lost to Agut. "I didn't do what I did for set and a half, which was after few balls stepping in and taking control.
"If you give him an opportunity, if you drop your level, he's a kind of a player, similar to David Ferrer, that is going to get on top of you, which he did.
"It's always good to have a test like this in a Grand Slam. You know, better earlier than later."
Next up for Djokovic is Lopez, who has had some outstanding victories already in this US Open. The Spaniard beat Mardy Fish in an emotional five-setter in the second round, before seeing off the challenge of Milos Raonic in impressive fashion.
Lopez then continued his good form, ousting Fabio Fognini, who has knocked out Rafael Nadal in the earlier round, in straight sets.
"It's hard to predict how the matches are going to look like," Djokovic added. "As you progress in the tournament the matches are getting tougher, which is logical to expect.
"I'm going to play Feliciano Lopez, who is also a Spanish tennis player but doesn't possess the typical Spanish style, back of the court. He comes in and slices, he is physically very strong, big serve. Hopefully I can be ready for that one."
Lopez might have managed some commendable victories in this US Open, but the big one, of course, is the quarterfinal against Djokovic. The left-hander knows if Djokovic brings his machine-like game on Tuesday, he will stand little chance of progressing into the semifinals.
"When Novak plays good, you don't know what to do to be honest," Lopez said. "He has no weaknesses in his game.
"I just have to try my best, take the chances, enjoy this match in the biggest court in the world against the best player in the world. This is what I have to do.
"In order to win I think I just have to be very aggressive and I have to play even better than I did today and the day I beat Milos. If I'm able to do it, which I don't know because the level is getting, you know, up and up every round I win. So, yeah, it's a great opportunity and a great challenge."
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Djokovic vs Lopez is the second match of the night session at the Arthur Ashe Stadium. The first match -- Serena Williams vs Venus Williams will begin at 7 pm local time (4.30 am IST, 12 am BST). Live coverage of the quarterfinal will be on Ten Sports in India, while the action can also be watched via online live streaming on Ten Sports Live.
Viewers in the US can watch the US Open 2015 live on ESPN, while the live stream option is on Watch ESPN.
UK viewers can catch Novak Djokovic's match against Feliciano Lopez live on Sky Sports, with the online live streaming option on Sky Sports Sky Go.
Audience in Australia can watch Novak Djokovic vs Feliciano Lopez live on Fox Sports, while the 2015 US Open match can also be live streamed on Foxtel Play.