Serena Williams passes Madison Keys test in last 16

Serena Williams will face sister Venus Williams in the quarter-finals of the US Open after beating Madison Keys in straight sets.

Keys looked to be holding her own as the first set remained on serve for the opening seven games.

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The youngster started the eighth game with an ace, but an unforced error and successive double faults gifted Williams a 5-3 lead.

Despite Keys forcing 30-30 and the possibility of a break back, the world number one was able to see out the first set.

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The 20-year-old faced an early break point in the second set, but showed resolve to hold as her powerful groundstrokes allowed her to stay in touch with the top seed.

However, a break in the seventh game gave Williams a sniff of victory and the 33-year-old broke again to seal her progress into the last eight with a 6-3 6-3 win.